Only the power of the dragon balls can stop me!
What would it be? Does not have to be a videogame series.
I guess you can have a Mario portion with heavy Sonic theming and a Sonic portion with Mario theming. Like both retrain their gameplay elements for their side of the story but Sonic can plow through Goombas in loops in a grassland while Mario can hit blocks while jumping on badniks.The problem I have with the idea of a Mario and Sonic crossover platform game is that the gameplay concepts between the Mario and Sonic games is far too different for me to picture any real way to make the crossover work, regardless of whether or not you have the crossover be a 2D sidescroller or a 3D platformer.