Alex, so far, I doubt would get the chance to be in, because he's shrouded in so much mystery. You don't get to fight him, and that puts him at a major disadvantage as far as moves go...
Excluding the oddball sworders Metaknight and Pit, so far we've got Link and that semi-truck Ike. Melee had four "standard" sword users - Link, Y.Link, Marth, and Roy. Link is link in Brawl, end of story. Marth and Roy have been combined into Ike for FE representation. Y.Link... might make it back in, might not. Anyways, that's 2-3 "standard" sword users. They have Lin as an assist trophy to rep FE again, instead of being a character... which means that last slot could very easily be filled by Isaac or Felix to represent Golden Sun.
And unless another GS character makes it in as an assist trophy, I doubt they'd get much more representation other than as costume colors. Red Isaac = Garret & Black Isaac = Felix; their armors are quite similar (at least from what I remember I haven't seen their character art in forever), and so a change in the color of hair ala Peach>Daisy (or Diddy>Dixie) along with a different armor color would easily represent those two... as far as the others go, it would be a long stretch... lol blue Isaac = blonde Marth XP
A Lighthouse stage would work wonderfully with Brawl in the direction it looks like Sakurai is taking it; many stages are open and simple and are prone to changes happening, so a Venus Lighthouse that splits open (exposing the... star? in the center, giving chance to be damaged by it/ fall down through the split) would fit perfectly.
I have no doubt in my mind that Isaac will make it. At the very least, there's going to be at least ONE assist trophy from GS... all those Psynergy that could be used... (lol, now I'm picturing Isaac summoning stones to circle around him for a neutral air)