Slime Master
Smash Ace
They could always go with the tried and true "the paid content is purely cosmetic" route. I haven't played Brawlhalla but my understanding was that it takes the MOBA route for unlocking characters i.e. rotating "free" characters, in-game currency can be used to permanently unlock a character of your choice, or you can use real money to do it faster; we could be looking at something like that. I'm definitely weary about it being F2P, but there have been non-predatory F2P games. We know so little so far that I can't completely write the game off yet.