Here's an interesting thought exercise:
Imagine a game where reproduction is the gimmick. I don't mean human reproduction: I mean things making things.
The gimmick isn't to be used as a backdrop: it's to be used as the primary game mechanic. The point is that the player's primary action is to change the nature of the thing that reproduces. Sort of like "battlebots", I guess, except that it's about making more bots rather than killing things.
There are a lot of more advanced ideas - resource management on a "racial" level. Cooperative, symbiotic, and parasitic activities. Changing environments. Etc. But they must all be managed through tweaking the "machinery" of the reproduction.
The difficulties in this thought exercise are twofold: How do you make it fun, and how do you make it at all?
What kind of system would you build to allow a player to create reproducing things? And make it accessible?
How would you do it?