Take the Mario and Luigi RPG series. Get rid of Luigi, replace with Sonic. Just add some sort of Bowser and Robotnik team up plot and there, done, awesome. Wind up with some sort of Super Mario Bros Z stuff here.
Literally for year's I've wanted to make a M&L:SSS hack like this...I'm just not good at GBA Hacking yet...
If ya wanna let things work, do what happened with Crash and Spyro for those two GBA games: Throw one character in another world or whatever, and let that world's hero deal with the other character's baddie as best they can. I'd love to see how Mario would handle Robotnik/Eggman's badniks and boss machines, and how Sonic would be able to handle Bowser's shenanigans. Let each character go through a set number of stages from the other character's game history, and let them each be able to fight bosses from the other character's series, just to see how they would handle that madness. To be honest, this is about the best way I honestly think things would go well that both Nintendo and Sega could agree on. But that's just me.
Yeah, now that I think on it, Mario and Sonic could do some pretty cool attacks with each other. Mario hitting a balled-up Sonic into an enemy with his hammer, for instance? Or Sonic doing his Blue Tornado from heroes combined with Mario's fire glove? Lots of potential.
Fixed. You know, this is actually a really sound concept. It would be absurdly easy to transfer Sonic and Tails' abilities into an M&L style game, especially thanks to Sonic Advance 3's team moves. As an example, they could easily mimic Mario and Luigi's first out-of-battle moves in the first game; Tails carrying Sonic across large distances a la Mario's Spin Jump, and Sonic leaping off of Tails up to higher ground a la Luigi's High Jump.
Also, many of the Bros. Attacks would fit perfectly; I.e. Tails flying Sonic up and dropping him in ball form into the enemy, with the !Advance! version swinging and throwing Sonic for more power, and possibly catching him mid-air and doing it again, for a possible infinite multi-hit a la Knockback Bros. Advance Or, even a Tails-Propelled/Enhanced Sonic Tornado...
