Have you considered working off tabletop mechanics? I am not fond of them myself, but the advantage in structure by, say, basing off a D&D system certainly has its merits. If you're into that sort of thing.

Only other system I have is built for my canon, which is a bit too much reading to stick into the usual roleplay, so I'll avoid getting into it here. I'm sure other systems exist, I just don't personally know many of them.