Well, it's always been known that crossovers only usually happen when a company or product is selling badly. An example of this happened a few years ago when the Rugrats met The Wild Thornberrys. A few months later, their showing were rarer and they stopped making more, or so i've heard.
Also, plumber meets blue talking hedgehog? I dunno. I like the idea, but I still say it's cheesey. It would be good in a Super Smash Bros., or in a sports game, but it could be good in any, if done well.