That would be pretty cool and I wouldn't mind seeing it but I would rather just pull certain characters from Smash over to Playstation Allstars. If just Link, Ike, and maybe Fox were introduced to this game then I would be set. No need for Captain Olimar
I highly doubt that would ever happen. The games are just way too different and it would be pointless. They're much better off just giving the other the OK to add a character into their own game. I wouldn't mind seeing a Nintendo character in All Stars, would prefer Falco myself since he's my main is Melee and Brawl, however Mario would most likely choice. Don't know what Nintendo would want in their Smash though from Sony's side.
By the way, just call me Count for short.