I don't think Nintendo and Sega are planning to do any 'epic Sonic crossover'. Just like I've given up hope that SquareEnix has plans for an epic Dragon Quest/Final Fantasy cross over RPG. However, the concept for Dimps (the developer of Sonic Rush and 'Sonic 4') and Nintendo teaming up to make an epic 2D Sonic/Mario cross over is sound. Not only is Dimps a good development group, proving themselves time and time again at being capable of making amazing games (Dragon Ball Budokai, Dragon Ball Advance, Street Fighter IV, most of the better Sonic games in the last 10 years), but they also are just a solid development group who shows they can work for a wide variety of developers (Sega, Capcom, Konami, Bandai, etc).
In other words, what I'm saying is, if Dimps and Nintendo teamed up to make a Sonic/Mario cross-over, I'd play it in a heartbeat.
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