Geno won't get in because he's owned by Square Enix and if Square Enix was to include a character from Smash Bros, they'd want either Sora from Kingdom Hearts or Slime from Dragon Quest since those are the two most iconic Square Enix characters from games that are currently on Nintendo consoles.
I'm still betting my money on Slime being the Square Enix choice because of how close Nintendo and Square Enix are with that series now. Unlike Kingdom Hearts which still appears on Sony consoles, Dragon Quest is exclusive to Nintendo consoles and Nintendo even publishes all the titles outside of Japan. Plus there was the recent Fortune Street which was a giant Mario and Dragon Quest crossover with Mario and Slime both on the front of the cover to show they're the mascots of each series. So yeah, I'm expecting Slime to be playable.
Isaac might have a chance but the recent Golden Sun sold pretty poorly which will probably kill it. The only thing that might help him is the fact he was heavily requested for Brawl which helped get him in as an Assist Trophy but the series lack of popularity in recent years is going to make it tough. Had Dark Dawn been a success I'd probably say he had a good chance but right now, I wouldn't bet on it.
And Mega Man is dead people, don't even try to get your hopes up. If Capcom won't even put him in their own mascot fighter game Marvel vs Capcom 3, and still not put him in Ultimate Marvel vs Capcom 3, what makes people think they'd actually care to get him into Smash Bros? Any Capcom character that gets into Smash Bros will either be Ryu from Street Fighter or some kind of character to represent Monster Hunter.