@MarshReaper replicating the hook of Smash seems unfeasible (your favorite characters smacking each other). Finding libre characters isn't hard but finding ones to fit the gameplay might - is having Tux the main appeal? I would look at what other clones have done and try to replicate them but with a twist.
Maybe the twist could be something to do with it being FOSS. Like you can "read the source" of the opponent, or modify your character's abilities per match.
Mix and matching sounds like a cool idea. Although, we will need to figure out balancing for this system as well.
@MarshReaper a rock paper scissors approach could be a good choice. If it isn't a core mechanic then yeah it's just extra complexity (not worth having to start with).