One of the problems in exploration games is that the player always comes in from "on high".
That is, the player has full control over their movement and can see quite far. Not only can they choose whether to engage or not, they have extremely fine control over their engagement at all times, and can change their mind as they like.
Two problems with this.
1) It negates the impact of most content.
When you can see the content at range and keep an eye on it the whole time you close, it doesn't hold much mystique. This is especially true if you're relying on variants, because variations don't matter at range. They usually aren't even noticeable.
2) It negates the engagement of the content.
When the player has such precise control and good vision, it is difficult for the content to reach out and engage the player. This means that the content will just be a visual unless the player chooses to allow it to engage. Even if your content is brilliant and engages in many nuanced ways, that is painfully blunted if the player has this much control over its vector.
To fix these problems, we need to adjust both the player and the content. Not to a new normal, though: we're not looking to simply reduce the control and vision. We want to vary it wildly, because then the player will engage the content in a variety of ways at a variety of times, which will give the content a lot of life.
Here are some methods:
A) Vary the player's vision
We can reduce the player's vision. This can mean things like day/night. It can mean things like fog, rain, etc. But in addition to global changes, we can also use local arrangements - forests, caves, interiors, and other topologies that get in the way. Add additional complexity such as flashlights, flares, different vision modes, and even spotlights from space ships or something... all of these will vary the player's vision and make the same content affect them in different ways. Limited resources in certain vision modes are another way to add stress to this equation.
We can also increase the player's vision in a variety of ways, such as "hunter sense", or sonar, or a number of other ways of overlaying added data onto the normal audio/visual experience. Completely non-visual inputs such as audio should be considered as well: a heavy rain can make it impossible to hear a growling mountain lion hunting you.
Suddenly changing the player's sensory ranges is also fun. The player opens a door or window and suddenly they can see quite far... and something five feet away can see you just as well. This can be more terrifying than stumbling into that thing, because it seems that much closer due to your long vision. Letting the player trigger these events is very rewarding.
Secondary inputs such as sensors, remote monitors, and so on are a big factor in some kinds of games - for example, using a satellite to observe the surface of a planet. Another thing to remember is that you can increase vision while decreasing control using glass windows, raised platforms, wire fences, and so on. Anything you can see through/across, but not easily move through/across. Giving the player an option to scan ahead at the cost of time/resource can be valuable as well.
B) Vary the player's control
Reduce the player's control. This can be accomplished with things like vertical arrangements - climbing, sliding, and so on definitely reduce your control while also screwing with your vision. Things like thermals, flowing water, and currents can actively drag the player along, which can be fun if your game is designed for it. In addition, trampolines, intermittent vents, rope, bungee jumps, and zip-lines can give the player the option to move in an unusual fashion if they like, which works well as long as that unusual fashion offers advantages but also forces content engagement (IE, a bungee jump into a pit with a huge, hungry squid at the bottom, or a zip-line to the crumbling tower at the top of an ancient facility).
Those are just the start. We can also offer fundamental movement modes that have restrictions built in, such as parachuting, gliding, sinking, skiing, etc. In order for them to remain explorative, the constraints on these are usually going to be that you can't go backwards, or that it costs resources to go backwards. That way you can still explore, you just can't explore freely. Alternately, these alternate modes could be non-exploration parts of the game while still exposing you to explore-mode content in a new and very constrained way.
There are also some ways to do optional control, where the player is allowed to do things, but knows there will be repercussions. For example, they can fly their space ship into the ancient ruins, but it's going to damage them and make the wildlife hostile. Or they can unlock all the doors at a security station... but anything on the other side will be able to get out!
Lastly, you can flat-out take fine control away from the player. For example, if the player accepts a mission, you can cut to them being on the quest location. This requires your game to have that as part of the core experience, to avoid making it feel overly forced, but the idea of being able to simply put the player in any situation you want is fantastic from a content engagement perspective.
C) Create content that varies/reacts to the player's vision
The content itself can play with the player's vision. An easy example is a building: no matter how close you get to the outside of the building, there's an interior you can't see, or can only see through specific windows. Something which physically blocks the player's line of sight but also guarantees some content within that blocking thing. Things like pop-up walls are also useful, as long as the algorithm placing them specifically guarantees something worth finding as well.
The content can fuss with the vision in other ways. Animals might have stealth capabilities or be flat-out invisible (at least to some wavelengths). Alternately, plants might glow and actually improve your vision. Glowing frogs, mist-spitting plants, and so on. A plant may explode with a blinding flash of light or a huge cloud of smoke. Vision is also not just vision: animals that growl, plants that chime, a facility might whine so loudly it makes you deaf, etc.
The content can also move to protect itself from or expose itself to visuals - digging into the ground, coming out during the day, only flying in the fog, etc. This requires some AI, but it can be very interesting, especially if the player can take advantage of those changes by lying in wait or planting sensors.
The last option is to have content that reacts to what the player can see. For example, monsters that freeze when you look at them, or take damage as you look at them. Plants that grow if you watch them. Animals that panic and run if you can see them.
D) Create content that varies/reacts to the player's control
The most obvious way to have content vary player control is to have content be big, so that the player has to navigate it. Buildings are the easiest example - not only will the player have to walk around if they don't want to engage, but if they do engage, you'll be in a set of rooms with closed doors. So that varies both control and vision.
Also, content can do any of the things talked about a few bits ago - trampolines, ziplines, current generators, gravity cages, and other things can be part of content. Plants that blast air at you to drive you away, animals you can ride, and so on. It's best if these things have both upsides and downsides and can be either useful or hazards depending on the situation. That creates a lot of good emergent situations.
Indirectly, content can affect a player's control by adjusting global parameters. For example, you can't fly through dangerous fog (fog is content), you can only ski on snow, you can gather jetpack power on atmospheric planets but not on airless ones.
There's also the seriously meta options, such as having a plant that sings and will make you fall asleep if you don't keep wobbling the control stick.
After all that, you still need to have the content matter to the player. The way the content interacts with movement and vision is only one facet of how it can engage with the player.
Most exploration game content "engages" with the player by waiting for the player to come up and harvest it. There are a lot of better options, but you need to keep in mind the feel of your exploration game: being too forceful with your engagement is generally not great. This is why the idea of hostile content is annoying: I don't want to suddenly be in a battle with a giant monster or a military drone. Changing the gameplay mode without warning is just annoying.
The trick is to simply give plenty of warning. Animals don't just attack, and even hunting animals can be detected in advance by the player, who will see them in the bushes or hear them growling. Let the player choose whether to engage, push past, or route away. Of course, the player's exact situation will determine their choices. A player that ziplines into the middle of an angry hornet nest is in trouble... but they chose to zipline in!
Other than that, I would recommend you make most engagement systems adjustments, sinks, or fonts instead of forcing you to change game modes. Adjustments are things that affect your play - for example, a gravity ball that draws you in, or a wall of glowing lichen in a deep cave.
Sinks and fonts adjust the player's resources over time, usually getting more severe the closer you get. Draining or regenerating health and power are obvious. If you arrange you game to facilitate this sort of thing, you can do a lot more - for example, signal boosts or interference. Frying your pikmin. Covering you in ever-more goo so you move slower and slide down slopes...
F) Vary and Hook
Once you have players engaging with your content, you can extend that by creating nuances. For example, modular cat-predators: they always skulk around hunting prey, but as you swap out their various limbs, you get different specifics. This kind of variation only matters if the players are engaging: varying the content is worthless if the player chooses to not engage.
State changes are great as well, since you can layer content states. For example, the player might be able to restore power to a building. The nature of the building and its content might change when you do this - but there's also an opportunity for other content to change/pop into existence as well. Animals notice the lights. Sentry bots wake up. The elevator into the depths starts working. A radio signal broadcasts and someone hears it.
Layering and varying content adds much more life to the same amount of content.
You can also add in hooks by adding story elements, NPC commentary, and construction opportunities.
The easiest hook is safe zones: if there is some resource the player needs to regenerate (electricity, fatigue, etc), you can make specific content locations the only ways to get those. The specifics and the surroundings really start to matter!
Whether you're manually laying the content out or you're using an algorithm, a bit more care has to be taken in order to do these things properly. How you arrange them matters quite a lot: the meat of this experience involves how various content and constraints combine.
IE, a building alone is going to be dull. However, a building at the top of a rock spire surrounded by air-blasting plants that push you away... that's more interesting. Partly, the player has to figure out how to navigate the challenge. Kill the plants? Throw rocks to pre-trigger them? Using a jetpack? Land on the top of the spire with your amazing skill? Or all of those options one after the other!
In addition to the challenge, the player is being affected by the mystique. The limited navigation and vision means the building at the top remains interesting and engaging until the player reaches the top. At that point, the building's variations (windows, locked/unlocked doors, content contained, outlying modules) immediately matter to the player and they have to soak that up all at once at short range.
Anyway, that's what I've been thinking about recently. If you have any opinions, let me know.