While I know where you're going with that, a suit change would require way more spriting than an attack effect. And besides, Mario DOES use a lot of fire in Smash games. This way Luigi could have an electric shockwave or something, just to include that ability of his in the game.
And if not a fire stream, maybe something a little more physical, to better differentiate from the Final Smash? Actually I think that is for the better, but I can't really think of anyth- Mario's fists set on fire and he slams the ground (first hit) creating two quick but short bursts of fire going both forwards and backwards (second hit). If caught in the initial hit, the opponent is trapped until sent flying from the fire blast (though not that much more powerful than his side smash).
And if not a fire stream, maybe something a little more physical, to better differentiate from the Final Smash? Actually I think that is for the better, but I can't really think of anyth- Mario's fists set on fire and he slams the ground (first hit) creating two quick but short bursts of fire going both forwards and backwards (second hit). If caught in the initial hit, the opponent is trapped until sent flying from the fire blast (though not that much more powerful than his side smash).