First, I have to say there is no news about this. I was just thinking what it would be like. Also, Sega has made a fighting game with Sonic characters. It's either for the Saturn or Dreamcast (not sure). I saw it on X-play bundled in a Sonic collection.
I think it would be a good game. Fast and furious. And, Sonic games are fast. But, what other characters would be in the game besides Sonic characters? Because SSBros has not only characters from games Mario was in but, also other games that are only for Nintendo (except now they have Solid Snake). I guess to find potential canadates would be to look back at characters who have only been on Sega consoles before the next console came out (I put it like that because there are remakes of older games. Hope it makes sense).
One last thing. What would it be called?

On a sidenote , Battle Stadium D.O.N. is basically what you described, except it's a Smash Brothers clone with characters from One Piece, Dragonball Z and Naruto. Very cool looking actually...It was pretty big at Otakon this year. Too bad it'll never come out here because of licensing issues.

I guess you could take the cast of Sonic, throw in Alex Kidd, the guy from Shinobi, Vyse, some Phantasy Star folks, definitely some Sakura Wars members, some people from Virtua Fighter, Ryo (Shenmue), the guy and the girl from Outrun...It'd be fun, no doubt. Surprised more companies haven't thought of this.

Name: Sega Super Smash. Or Sonic Super Smash. Or Sonic's Super Sega Smash.

This game already exists.  For the GBA.  Despite that, it's supposedly quite good.

There's also Fighters Megamix.