This. I don't get why people have to fight over it. If Shulk gets in Isaac isn't any less likely or anything like that.
Also, not meaning to be offensive to Isaac fans, but he had his best chance Pre-Brawl, when his games were relatively new and well received. Now, I know Dark Dawn came out after Brawl, but it wasn't met with the best reception...
Also, the Golden Sun games haven't aged very well at all. There's an entire Lets Play about how bad Golden Sun's story is (
http://lparchive.org/Golden-Sun/), and the Gameplay is that of any standard JRPG.
Not saying this affects Isaac's chances or anything, I'm not against his inclusion at all, I mean, him having an Adult Isaac alt with that glorious beard would be amazing enough, but I don't see him as likely or anything like that. I can only see him as DLC more than anything.