Isaac sits in an awkward situation. Golden Sun received a new piece of content, but Isaac's trophy wasn't brought back for some unusual reason. That does put GS above Helirin since GS is one of only a few franchises without a stage to get a single music track in the game, let alone two.
Regardless, there's a different reasoning for Isaac being a likely candidate. Camelot is definitely working on something. We don't know what, but they've stated they can't do HD games (at least, without help), and I highly doubt Nintendo would bother giving Camelot additional dev team assistance for a measly sports game. Golden Sun is honestly the most likely thing Camelot could be working on right now, and if GS4 is in development, that automatically makes Isaac an extremely likely candidate (yes, he would appear in the game because Isaac was involved in the Dark Dawn cliffhanger as well; he HAS to appear in GS4).
If its not in development there's no chance at all, but odds are definitely in favor of GS likely having a new game coming up, especially compared to other franchises. Helirin not only hasn't had a game in forever, he doesn't have a trophy in the game at all. Isaac doesn't have a trophy, but GS does have two music tracks. Barbara is basically about the same as GS content-wise, except with one trophy and one music track.
The evidence in-game does not really favor Isaac. What goes in Isaac's favor is the more likely odds of GS getting a new game, because its in the perfect situation right at this point in time. Unless Camelot is doing nothing (extremely unlikely) or is working on another 3DS sports game (also incredibly unlikely), the only other possible thing they could do is Golden Sun, as Golf/Tennis and Golden Sun games are the only things they've done for the past 15 years.