What do mean "gameplay"? Are you talking about a video game or a TV series?
There is a Mario and Sonic video game coming out this Christmas. As for a TV show starring both characters in tandem, I honestly don't see that happening.
No it wouldn't because Sega had it's own game system at the time and Sega and Nintendo weren't working together back then. There's no way that Nintendo and Sega would have put aside their intense rivalry to collaborate on a Mario/Sonic TV show. As we know, getting rival companies to work together is like pulling teeth-very hard!