I'm into conworlding, and I've cooperated with my cousin on creating role playing games set in our collaborative conworld. Mostly we just use the d20 system, but we also experimented with d12 (even though it's not open like d20 is). We found the d12 system to be an excellent fit for our magic system, which freely combines different schools of magic (like elements) to give the player more creative freedom, rather than using set spells.
I also worked with my brother on a Gundam RPG, but we didn't do much with it.
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"If knowledge can create problems, it is not through ignorance that we can solve them." - Isaac Asimov