Haha, that's awesome!
Back when Melee came out my friends and I always made up our own character ideas for Smash Bros., and I ended up making up an idea for Secret of Mana characters of course!
My idea would have been playing as all three characters, but you swap through them. Actually the Pokemon Master in Brawl ended up being pretty similar to what I had in mind back then. The down + b move would call Flammie in and he'd pop in doing a little bit of damage, and pick up the last character and drop off a new one, so you'd cycle through Randi, Purim and Popoie.
I can't remember the movesets I thought up, but Randi was going to use stuff like the Sword, Axe and Javelin with the Whip for recovery up + b, Purim was going to use her gloves and have some nice grapples, maybe her up + b recovery would be a lucent beam that she kinda jumps along like Pikachu, Popoie was going to be very light but have lots of distance stuff like the bow and boomerang plus some magic.
I used to spend hours thinking about that stuff, haha. I wish the Mana series could have a smash style battle game, it'd be awesome.