Face it, the idea of the descendants failed, that's one of the reasons why Dark Dawn wasn't that good, I think we need to just stick with the original 4 in each game, they are the 4 most iconic protagonists of the series, Isaac, Garet, Ivan and Mia, just have them in Golden Sun 4.
As for Matthew's team, well here's an idea, the beginning of the game starts out that they set out to do battle with Alex himself in a scripted battle where they lose, kind of similar to Saturos and Menardi at the beginning of the original Golden Sun, thus Alex kills off the descendants from Dark Dawn, which is Matthew, Karis and Tyrell, the 3 seen at the end of Dark Dawn, and of course after words Alex vows to finish off all of descendants of the Adepts of Weyard, thus confirming Alex to be a villain, I don't know, maybe I just want to see Dark Dawn's protagonists killed off by Alex, lol, bring back the original Golden Sun cast.