Wait, If he has zero factors for said reasons, then how does isaac have any? I'd like to know so I won't get any hopes up until he gets announced. (years from now; at least I'll have GS3 to wait)
Hence the "possibly." I always forget about Isaac personally, probably because I haven't played Golden Sun yet. But yeah, he's very likely, even if Golden Sun isn't as big of a name as Punch-Out!! yet.
But, in series like this, where the main changes from game to game, I think its pretty clear that most of the time we've actually been getting the newest representative of the series as well. For Fire Emblem, we got both Marth AND Roy. Later on, we got Ike, who was the newest at the time of development For Mother, we didn't get Ninten, but rather we got the most recent character Ness, and then when a newer character came out for that series, we got him as well. PT doesn't fully fit this trend, since we probably should have gotten a DP trainer (Even if DP's infromation probably wasn't fully known to the Brawl staff during development) Then again, PT is somewhat invalid due in part to him taking three games to show up. Another legacy character though that fits is Link, who also shows up in both his most recent incarnations. In the end, I really think you're disregarding the importance of a character being new, and of the little precidence we have of series that have both old and new legacy characters, we usually get the newest irregardless if we do or do not get the old one.
I really hope you'd notice someday how the inconsistencies in your theories ruin them most of the time. Anyway, let me break it down for you:
EarthBound had Ninten and Ness during Smash 64's development. Ness is the most popular character in EarthBound. Sakurai adds Ness. Later adds Lucas, the most recent character.
Fire Emblem had Marth, Alm, Celica, Sigurd, Celice, Leaf, and Roy during Melee's development. Marth is the most popular character in Fire Emblem. Sakurai adds Marth. Later in Melee's development, Sakurai sends in the clones, including Roy (the most recent character). In Brawl, we get Ike, the then most recent character.
Pokemon had Red, Leaf, Gold, Kris, Brendan, May, Lucas, and Dawn during Brawl's development. Red is the most popular trainer. Sakurai adds Red. Keep in mind we're focusing on trainers, this is a different category than the Pokemon themselves.
Notice something?
The first character added is the most popular, the "face of the series" if you will. Most of the time, that character is lead of the first game. Ninten is the exception so far which only means that a series is CAPABLE of having a "face" that isn't the original (see: Final Fantasy).
The characters added AFTER this lead important guy is always the most recent. Lucas is recent. Roy and Ike were recent. If we got a new trainer, he (she?) would be recent. I haven't mentioned the Pokemon themselves, but Jigglypuff, Pichu, Lucario? All recent at the time of their inclusion. For the record, Mewtwo doesn't fit under this because he was supposed to be in Smash 64 (and since he's part of the regular 20 in Melee, it seems Sakurai considered him important enough to fully implement).
Short version: unless this new guy usurps Isaac's position as most popular Golden Sun character, he ain't getting in over him. His only chance is if GS gets two characters, and that in itself is very, very unlikely.