I agree.  The only way I could see it working is if they're able to come up with a new gameplay formula that captures the spirit of both franchises but at the same time is different enough that it's seen as an entirely new experience compared to a traditional Sonic or Mario game.

Super Smash Bros :D

Colors and Generations are actually pretty decent! :S

I still don't like them being together and hope that one day Sega makes a new console and puts Sonic back where he belongs.

I really don't see how a Mario and Sonic action game could work. Sonic is just too much faster than Mario. In order to put them in a game together you'd have to give them separate levels. And if you're just going to give them separate levels then what's the point? Why have them in a game together at all? Doing it just for the sake of doing it? I'm not a fan of that.

If I were in segas shoes, I'd be trying to eliminate other popular franchises too - sonic in call of duty, sonic in grand theft auto, sonic in Overwatch...
It's the perfect strategy.

A platformer where both sonic and mario work well together in? I have a hard time imagineing such a game.
Imo trying for such will just result in a bad game.

^ thank you! glad to see Im not the only one of that opinion.

What the heck Miyamoto. That sounds like a cool idea. They should have definitely made it happen already.

Hmm, if it is in 3D, I don't think it is impossible.

The problem with most of the 3D games is that they lack the speed Sonic is know for, meanwhile a number of Mario's more recent 3D outings have picked up speed and in case of the Galaxy games, included some more physics based gameplay. The issue with combining the two together is that such 3D games don't really lend themselves to having multiple characters unless you are having them switch out on the fly and a 3D world or NSMB game wouldn't let Sonic zip along unless it was online or on the Wii U where each player had their own screen.

I firmly think it is possible... but highly unlikely because it would take too much effort to create. A Kirby crossover with Nights would be more practical to get off the ground then a platforming game with Mario and Sonic. The best bet for a non-sports, non-Smash Bros. crossover with Mario and Sonic would be a RPG style game similar to Mario and Luigi or Paper Mario's first two games.

That's subjective.

But I already mentioned why in your Sonic 2Dvs3D thread: http://gamrconnect.vgchartz.com/post.php?id=6050378