Well since Sonic joined in the celebration of Mario's 20th anniversary...

[Kuzu]

We got Brawl people, isn't that enough? I mean you can make them fight to the death, isn't that what we always wanted?

[Spookers]

I looked up what this was just now. I tittered. Try harder.

I think they make a cream for that now.

Mario's not really slow so much as Sonic is fast. Perhaps even switching worlds like in Crash Purple and Spyro Orange would be a good idea. And as far as Mario's getting hit once and dying with Sonic's ring-based system, they could just pick one for both characters... Not really a problem.

Mario's got power-ups. Sonic's got Speed. However, Mario could get a new power-up (They're making 'em left and right now, anyway) to let him keep up with Sonic in certain parts. Another solution is to give the characters stats, such as Sonic's really fast, but he can't perform certain abilities that Mario can.

[somesortofrobot]

I'd rather see an update to this picture.

[KrazyBean]

Mixing water and oil?

Make an Emulsion. As Azukara said, make a Sonic Unleashed-type game where you swap back and forth between Speedy Sonic and Platformy Mario.

A lot of people enjoyed the Werehog, even though its design was mediocre at best. If you swap out the Werehog with Mario alongside some good Mario-ish level design and you're golden (and it will sell by the millions)

If the level design was like Sonic Colours and Super Mario Galaxy COMBINED...then Day 1 for me, also the Sonic Unleashed idea will definitely work