Recently I have been musing on merge games for obvious reasons :P
Extended from that would be user provided content as a merge result. The core features I can say would add variety which could not be computed would mainly be Art and Title, maybe a description.
The problem is the moment I add game features like attack, defense, or maybe something like earning multipliers, then frankly if you ignore art and title then there is no reason for users to submit their own.
I looked for example at having user defined Towers in a Tower Defense setup and it makes no sense to allow user defined content because you would need to have the logic for the actions coded anyway.
If you have all the logic for abilities and actions coded anyway then the game can generate its own variations just through simple probabilities.
The above image is a sort of breakdown, now if a single Tower can only be 1 of the 3 attack types and likewise have 1 of the 7 effects you effectively only have 21 types of towers.
Now this can be more agnostic and maybe you are only ever allowed 2 Towers but each can have all 3 attack types and each type can have up to 100% of each effect.
Does that warrant enough variety to make it worthwhile for a user to submit a Tower?
Not really because it is defined variety essentially and the system can compute them all, no matter how much variety is added it does not really add complexity.
If anything if I were to now combine this with a merger system the variety merely extends the time it takes to reach Max.
If I change my thinking from how can I make it worthwhile for users to submit content to how can I have users add complexity that I could not through computing.
It is all fine and dandy telling people oh you can add your own Tower design, since I could not possibly go through all the artistic styles possible with user creativity. Same for titles, but these things do not add complexity and therefor are not satisfying.
The only example of user created content that has unlimited complexity is level design. Something like geometry dash:
People build their own levels using predefined building blocks, the level as a whole would be the user defined complexity built on top of predefined logic.
Simply put, that is fucking madness.
I personally am not going to even attempt something like that, so a new criteria is: "What is the simplest complexity that a user can add but I could not have computed."