I have a few ideas of my own:

A dude (or dudette) with an earth semblance, and maybe s/he has gauntlets like Yang, but maybe they transform a friggin grenade launcher.

A dude based off Afro-Samurai with a dragon semblance, he has a katana (that, idk, turns into a shotgun), and can summon a mighty dargon.

A chick I used in another RP that had lightning nunchucks.

And maybe a ninja based off Ibuki from Street Fighter. :^)