Well, if it were to happen, I think it'd be because Matthew's just easier to do. Basically, since he looks so much like Isaac, it'd probably take a lot less work to make Matthew the echo. Not saying it would happen, but it sort of makes sense to me.
The problem would probably be how to differentiate Matt's personality from Isaac. Going by how people talk to him, particularly how Karis is always backseat leadering him, it would seem that Matthew lacks a bit of his father's confidence. I suppose they could just make Matt more soft-spoken and unsure of himself than Isaac. This is just my view of the character, but I'm just showing how it'd be pretty easy to make him stand out from his father.
The way I see it, Matthew would be easier to do, but Felix would make the fans happier. If they were to, say, go out of their way to make Matt an "Alph-like" costume of Isaac, I don't see why they wouldn't just use that model they already made as Isaac's echo. I feel like either Matthew's not in the game at all, or he's going to be an echo. I can't see them basically making three playable Golden Sun characters all at once (even if Matt was treated like a costume).
This isn't a popular opinion, but I'm just saying. I think Felix is more likely for the echo spot, however.