I have doubts about a Golden Sun character, seeing as it's really only outside Japan that one has heavy support, and we've already got plenty of characters that are like that, such as Ridley and Little Mac.
That, and I feel with Camelot pushing Matthew as the "main" hero of the series, even going as far as stating that the first two GS games were mere "prequels" of the main story that starts with Dark Dawn, they would personally request Matthew instead of Isaac. However, a compromise could be met, that with Matthew's near identical appearance to Isaac could lead to an alternate costume of young Isaac's outfit.
I am not trying to use the relevancy bs, but rather the fact that Camelot is a 2nd Party, and 2nd Parties usually have a bit of say in their characters (I.e. Lucario and Ike)