This one. (it's actually incredibly popular in r/AndroidGaming these days)
I was thinking single player, at least initially. Multiplayer would introduce quite a few complications... though I don't think it would be too hard to bolt on support for it later, as long as the system's architecture is sane.
Thanks for that resource! I've never heard of a Z-machine before... I'll definitely look into it. It would almost be like knocking two birds out with one stone if I could implement or use one.