I love player-created content. I love it so much, I don't even want to call it "mods". I want it so flawlessly integrated into the core game that it's just how you play.
From a technical standpoint, this isn't as difficult as you might think. Just pipe player creations into other players' worlds. Sure, there's issues with quality or suitability, but the technical act of taking a player's constructed content and showing it to another player is pretty straightforward.
For me, the problem isn't the quality of the content or the technical aspect of sharing it. The problem is the disconnect between the players. The context of the content is lost.
Players are happy to create content, but a lot of the time, they create a context for the content that doesn't exist inside of the game world. For example, I'm playing Space Engineers. Sometimes I just want to create a functional ship - hey, look, this drills really well. But a lot of the time, I have a fiction in my head: this ship is a cryo scout ship with a crew of 10, this station is for several alien species that can't breath the same atmosphere, etc. These fictions are interesting to design for and, in theory, could give rise to a lot of interesting things happening with them.
If the ships and space stations are simply injected into another player's game, the context is lost and they are reduced to a jumble of parts ripe for the harvesting.
The context is removed, but it can be rediscovered. It can be hunted for. I see this a lot with Let's Players doing community showcases, exploring ships with only a bit of a text blurb to tell them what's up. This mystery has power - perhaps especially because there is little validation. You'll never know whether your guesses are right. Maybe you guess the station is a hospital for contagious diseases instead of a multispecies diplomatic station. In some ways, that doesn't matter: as long as your guess was interesting to you, you're having fun with the content I built, and that's the key.
In some ways, it matters a lot.
If the context is solid, it can live on and grow as the game progresses - and the new player can even be invited to participate.
If a new player understands that my station is a multispecies diplomatic base, they can quickly realize that the other ships in the area belong to these species. Suddenly, the fact that none of the nearby ships have weapons makes sense, and it's a reason for that player to design a weaponless diplomatic ship themselves. If one of the species is fronting a battleship, suddenly the player knows that there's an overly warlike species disrupting the negotiations. This steadily unfolds into a web of contexts and small scenarios.
We're talking solely through space ships existing in a specific place. All of that context can be built and explored without any combat modeling, or diplomatic modeling, or even any concept of time moving forward.
The core gameplay of most of these space ship building games is combat, or perhaps resource wrangling. What if, instead, the core gameplay was about creating and relating scenarios?
One way to do this would be to have the core gameplay be about designing ships for various groups and then watching them grow or shrink as your ship performs well or poorly. It is possible to make this complex enough to avoid dominant design strategies.
Another way to do this would be to rely entirely on player contextualization.
Players can present challenges. These might be blurbs such as "the nearby stars are highly radioactive, but colonization proceeds" or "a disease is spreading through the Traxican people, turning them into violent zombies!" Optionally, it can include worlds, stations, ships, people - as much as they want to put in.
Other players can attempt to approach these challenges by designing a ship and giving a blurb back. So players might design a colony ship to colonize those radioactive stars. They might design a medical ship to cure the disease, or an anti-personnel ship to kill the zombies.
The player that presented the challenge is required to use at least one player-submitted ship as part of their next challenge. So they find a colony ship they like and the next blurb is "in orbit around the planet, the radiation shields give out and the crew crash-land. They have to cobble together a base!" Or they use both the medical and anti-personnel ship and write a story about how the two clashed - and now the species are at war...
This freeform contextualization can also be done solo - creating ships to solve your own problems and creating new challenges based on that. While it seems masturbatory, it can be used to do worldbuilding in an organic and interesting way. The result is a pile of content ready to be repurposed for another player's experience!
This simple mechanism automatically includes quality filtering, since all the content is judged and propagated or ignored by other players. But it has several downsides.
The first downside is gameplay. You need to carefully design your gameplay so that players can get a feel for the ships and worlds that they live in. That means, at a minimum, the ability to embed commentary into designs such that as a player moves through them they hear explanations or see examples. My first impression of that would be allowing the players to record "ghost logs" like from System Shock 2 - you walk into a room, and you see someone slip across the room and lie down in a bed. Micro-machinima for context. This requires some effort - animations, recording, etc.
More gameplay would be better - the ability to use the ships. Some kind of statistical propagation as I discussed last essay. It's all quite doable, but you need to be aware that these gameplay methods serve to give the player context, not to actually be core gameplay.
Another thing that is troubling is the tense of the stories.
Since every story is narrated by a player, fundamentally everything "happened". The players don't get to unfold stories in real time unless they happen to be the one writing them. If you see a cool ship, your exploration of it is to see where it was and what it did, not where it is and what it is doing.
In order to avoid this, you have to allow players to create a "race track" of a scenario. Our scenario about settling a radioactive star would have various distances to travel with various amounts of radiation hitting you, attempts to scan planets while boiling under the sun, and then dropping the colonist base to the surface unharmed. More race track elements could be injected if the player is creative: crew members going rogue, the planet having ancient defenses, etc. Players could attempt these challenges with their own ships, or with any other ship submitted.
The problem with this approach is that the required gameplay can be quite bizarre. Moving from spot to spot, processing resources, and shooting at things are all pretty typical, but what kind of gameplay is "a crew member goes rogue"?
You can invent a gameplay for it. You can come up with something, I'm sure. Can you come up with gameplay for literally everything anyone will ever think of? "Ancients give one crew member psychic powers". "There's shapeshifters on the planet, and they pretend to be the crew". "The crew runs out of clean clothes". Each of these can be modeled, but each one is another special case, and needs to be thought up before anyone decides to use that special case!
I think the best way to do this would be to have a bunch of viable generic tasks that can be customized, but if there are new tasks, they are handled using the same context system. Your ship is intended to colonize a radioactive star, and in the end the other player decides how much he likes your ship... why not have him decide how well your ship can handle the rogue crew member? Submit a blurb describing how your ship handles it, and he'll decide how it unfolds.
This does take the event "into the past" again, but it's only for things that can't be done live.
Anyway, thanks for listening. I have a lot more to say, but this is plenty long already.