Let's talk about stages in a video game.
Let's start by talking about the difference between an old game and a new game. The most obvious difference is that in a modern game, you're generally regenerated 100% after each fight.
This difference is not as straightforward as you might think, but it is a good place to start.
If we imagine the power level of the party as it fights through a stage, in a modern game the curve goes up. The party gets stronger as they move through the stage, with occasional dips for minibosses and heavyweight encounters.
If we imagine the power level of the party in an oldschool game like Doom or Final Fantasy 1, it goes down. Occasionally it might pop up a bit if you find a resource cache, but in general the encounters wear on you.
This difference... is unfortunately not entirely straightforward. For example, in Final Fantasy 1, you actually are gaining power as you fight enemies. It's just that you're gaining long-term power while losing short-term power. This is a typical RPG situation.
So let's consider stages in a slightly different light: as treadmills or one-offs.
Treadmill stages are stages that can be farmed. For example, in an RPG you might exit a dungeon and re-enter it to farm the enemies. Or you might just get into umpteen million random encounters. This grinding is very much like an MMORPG - the stages are not really part of the plot, they're just parts of the world that maybe have some handwave towards being part of the plot.
It doesn't matter whether your power level goes up or down as you get into fights in a treadmill stage. Instead, what matters is how much time you spend to raise your long-term power level, and how unique/useful your level-ups (equipment, loot, etc) are.
One-off stages are stages that you can only visit once. If they are part of the world, once you've gone through them they are typically husks without any particular challenges. The point of a one-off stage is to squeeze as much out of them as possible in the limited amount of time you have with them.
Nearly all shooters are like this - you want to find the various caches and secret collectibles scattered throughout the stage, and you only get to go through the stage once. FTL has an interesting take, where you have to plot the least efficient path through the sector you can, while still not getting caught by the marching wall of death that moves ever right.
One-offs can also squeeze in the opposite direction, where you need to get through and hit the objectives while losing as few resources as possible. This is exceptionally rare outside of survival horror, but it is possible.
There are a few other pieces to designing a stage. So, let's hit them quick.
1) Is this a one-off stage or a treadmill stage? If a one-off stage, the stage can be built to have a progression (pieces of it explode, collapse, etc) while if it is a treadmill stage it has to remain relatively static.
2) Is the stage the challenge, or is the stage where you prepare to meet a final challenge? You don't actually need a boss if the stage itself is the challenge.
3) Is the stage window dressing, or does the aesthetic/conceit of the stage actually influence the nature of the stage's challenges and progressions?
4) Are the encounters bite-size servings or are they part of an ongoing challenge? If they are part of a cohesive challenge, they need to have long-lasting effects.
5) Is the stage progression interesting enough that the player cares? This is both the variety of the challenges and the way the plot unfolds.
6) Can the player interact with the stage itself, or is the stage just rails the player travels down? This means - can the player express himself by interacting with the stage? Not can he open doors or flush toilets, but can he actually change the way the stage plays out some? This can be taking alternate approaches (sniping from the rafters vs rushing in) or in altering the stage itself (using rockets to blow away walls like in XCOM).
There we go. Next time, we'll talk about party composition!