Throughout the 90's, it was a dream of many children to see Mario and Sonic, the two largest and most iconic gaming mascots of the time, cross over. We've since gotten that in the form of the sports titles and more loosely in the form of the Super Smash Bros. series, but we still have yet to see a proper adventure featuring the two of them. And frankly, I think the reasons that Miyamoto and others have given for it aren't particularly compelling.
We're about twenty years removed from a Mario and Sonic crossover really having any cultural significance, even if it did happen. But I would still like to see what they could come up with if they gave it a proper attempt.