Following Isaac, who else would you want to see in Smash from GS?
The nice thing about GS, in my humble opinion, is that the characters (with the exception of Felix and Isaac) are all different enough that they could each be unique characters and not clones. As for me, I don't think I'd care: write the character names on sticky notes, put them on a dart board, close your eyes and throw a dart. I'd probably be cool no matter where that dart hit.
That's a tough one. The best way to narrow it down is to choose the which psynergy class would be most unique to smash. With Isaac in, we wouldn't really need any more Venus adepts. A lot of characters utilize fire-based attacks, so I would exclude Mars adapts. Wind manipulation is a unique attribute, but lightning isn't as much, knocking out Jupiter Adepts as well. With both Squirtle and Ice Climbers out, there is a hole in the water/ice themed fighters (besides Greninja, but it mainly uses water in blade format, leaving a lot of room for other uses). So I would go with a Mercury Adept; either Mia, Piers, Rief or Amiti.
Isaac fills the strong warrior role pretty well, so I would knock out Piers from the list. A glass cannon spell-caster or a nimble speedster would be a good alternate character types so the other three would work. The original Golden Sun is my favorite of the series, so I would lean toward Mia over Rief. So it is down to Mia or Amiti and I am not sure which I prefer. I think I would have to lean toward Amiti, due to him being more combat-focused and being more of a direct complement to Isaac (fast and nimble vs. strong and sturdy). Plus I think he has one of coolest character designs in the series.
So this game's winner is...