"Here’s what Naka had to say about Mario and Sonic co-starring in titles based on the Olympics and the origins of it all:
   
        Actually, I wanted Mario [and Sonic] to appear together, so one time I went to give Miyamoto-san a presentation on how I want to create an action game which is co-starred by Mario and Sonic. It was in the GameCube days, and I took around four planners with me. It is a pity that the discussion was ‘clearly it isn’t necessary for Mario and Sonic to co-star in an action [game]’ and it didn’t become a reality..... I think I’m going to give another presentation so that Takashi [Iizuka] finds the necessity [of a Mario & Sonic action game] (laughs). "

       That would be so awesome! Ever since the first Mario & Sonic at the Olympics game was announced, I wonder if we would actually get a proper Mario & Sonic game. There's so much potential for such a crossover, it needs to happen! 

Link to the article: http://nintendoeverything.com/yuji-naka-still-wants-to-see-mario-and-sonic-co-star-in-an-action-game/

I think they should focus on making Sonic great again.When they manage that, I totally agree that they should pursue this.

It would be very difficult to marry both styles of gameplay. Though i'm sure that if they used something like Sonic Jam's 3D section it could work.

Though i'm impressed Naka tried it and Miyamoto is responsible for us not having it. For shame!

I'm willing to bet you'll be able to play as Sonic in a Mario game through Amiibo. Unlock Sonic on New Super Mario Brothers NX!

They're just different experiences... Mario is very much a platformer with tight controls while sonic is mostly about speed and really doesn't translate to 3D well.

They could have separate levels for the two like generations did or adventure. 

As long as the gameplay is 100% based on Mario games, then sure.

God damn it Miyamoto!

They just want Nintendo to make a proper modern Sonic game, since they've failed to do so themselves.