a completely acceptable thought, and one that i will more than willingly choose to look into. Most console systems only allow for four players at a time, which is often more than any household ever uses at a time. So having 3 characters at a time is a pretty good estimate of feasible character options.
The other characters I will discuss in a list before tackling them all one after another:
Sawada "Tsuna" Tsunayoshi - Tsuna has rockets on his hands that allow for him to fly and move like a jet. It also allows for him to attack with fire, blasting flamethrowers at his enemies, and trapping enemies in fiery pillars.
Lambo - A baby wearing a cow suit that can switch places with his ten year older self on occasion. When he is a baby, he throws pink grenades, while as a teen, he runs into his enemies like a charging bull, with electricity coating his body to increase the damage.
Mukuro Rukuro - This character can also transform. He can move around as an owl, though probably much more rudimentary than Tsuna, and change to a warrior who slashes with a trident. He has the power to cast illusions however he wants to make whatever he wants to happen, in your mind.
Takeshi Yamamoto - Yamamoto is a swordsman that uses the surrounding water to increase the destructiveness of his strokes.
Hayato Gokudera - Gokudera uses a launcher equiped to his arm to fire blasts of energy to destroy his targets. He can equip power-ups to give his blaster new powers. This goes from five different categories: Storm-His normal launcher, Rain-weakens enemy barriers, saving firepower, Sun-Works like a machine-gun, firing a barrage of shots that all home in on the target, Cloud-a shot-gun like attack that sprays an assault of energy over everyone in front of him, and Lightning, which was his strongest attack and could demolish anything in its path.
Tsuna is a speed guy, a lot like Sonic, only he is also aerial, so his game mechanics are often like that of a jet plane, moving through a series of obstacle courses. But he is also a humanoid character, and this is a combat styled game, so he still needs to be able to fight, as well as fly. This is where the joist stick will usually be used for directional movement, and if this is put on a Wii, which is probably will due to its origin, it will have access to the Wii motion controller, allowing for directional tilting. Think Super Mario Galaxy with that bird that carried Mario around, and you simply had to guide it using the Wii remote. You could then bring the arrow buttons as combat directions for when he fights in one place.
Lambo is an interesting method that I can only think of two ways that really did it similar. the Minish Cap from the Zelda franchise, and the mini-giant world in Super Mario 64. both of these worlds showed ways that the player could change from baby Lambo to teenager Lambo.
Mukuro is like a cross between Solid Snake and a Black Mage from the old FF. He uses Illusions to make himself invisible for stealth, send barrages of magical attacks that cause mazing theatrical effects, to other tricks for game play.
Yamamoto's Game direction is a little off, as all the games are heading towards the same way, but Yamamoto uses water to increase his attack, so he could damage pipes to let water out on the ground. The water could follow Yamamoto, just because of magic, but he could end up waisting water the more high-powered water attacks he uses. So it might require strategy in how much magic you use, and how often you just use basic hack-and-slash attacks.
Gokudera was the one I was inspired to amke a first person shooter. And I already explaine dhow his system would be built on power ups, but that isn't all. Throughout the game, there will be mobs that all seven characters will need to combat, maybe less for Tsuna, as he will be air-born a lot, but the enemies will use a one out of seven magic types that any of them have. Now, not many will use projectiles outside of Gokudera, who will probably get many of them, but still his fair share of close-up enemies that he will need to escape and regroup. But this Magic type will mean that sometimes Yamamoto's Water magic will destroy an enemy, while another will require two or three more hits.
The seven types of magic, with their core character are as follow, with their direct weakness:
Tsuna-Sky-weak to Sky
Lambo-Lightning-weak to Storm
Gokudera-Storm(Primarily)-weak to lightning
Yamamoto-Rain-weak to sun
Ryohei-Sun-weak to rain
Mukuro-Mist-weak to cloud
Hibari-Cloud-weak to cloud
Now, remember when I talked about the different power ups for Gokudera's blaster and how their types now seem to match up with the elements just shown below? well Gokudera can equip different types of blasters to change his blaster's type, but his equip blasters have limited ammo. this is why he is allowed these power-ups. because he uses strategy to choose which powerup to use. Does he just trust the always there Storm blaster to take out the weaklings one at a time, or does he round them all up and them unleash one cloud attack on them to shut them all down at once. strategy.
Of course, his power-up system and Hibari's weapon equip system will need to be looked into for the brawl section of the game as well afterwards.