Isaac is one of those character I have wanted since pre-Brawl. He seems to scream uniqueness in both move set and play style. In all honesty, I am surprised he wasn't playable in Brawl but there are certain things working against him.
1. He doesn't have a huge Japanese following. While he is popular in Japan; he has a much larger following in the West. Golden Sun, in general, seemed to do better outside of Japan.
2. Isaac is a 2nd party character. While Pokemon is also considered 2nd party, Pokemon is one of Nintendo's flag ship series. Golden Sun is a franchise that has only seen games released on the DS and isn't considered a fairly popular Nintendo franchise.
3. Golden Sun and Nintendo. As mentioned before, Golden Sun has a long history with Nintendo that started int he early 2000's but hasn't branched much. Golden Sun saw two games released for the GBA and a recent game for the DS. Lost Age an Dark Dawn was a seven year gap which is quite a long wait for a new game. Since Golden Sun is not a celebrated and highly popular franchise, relevancy becomes an issue.
4. While not a problem in Brawl, Isaac has run into competition with his younger son, Matthew. Matthew is Issac's son and, therefore, retains much of Isaac's abilities, psyenergy, and summons. Matthew is newer and may become the face of the series. I heard this somewhere but the first two games were created to set the story of the Golden Sun mythology and Dark Dawn would be the start of the major storyline for the series. This could mean that Matthew would replace Isaac as the face of the series.
However, Isaac still has a little fight in him. As of right now, isaac is still the mascot of the Golden sun series and thanks to his AT status, he is generally known in both the Nintendo and Smash community. Additionally, Isaac has a deep well of attacks/techniques/psyenergy to utilize for a move set. He could be a character that combines Venus psyenergy attacks with swordsmanship OR forgo the sword idea and stick exclusively to Venus psyenergy. Or if we want to take it further, he could switch between the two.
As well, Golden Sun has a lot to offer to the Smash franchise such as stages, items, music, ATs, and even incorporating the Djinn system which would allow players to summon monsters to help fight with them.
Isaac has competition from Matthew and, IMO, it's a 50/50 on who will get in. However, judging from his AT appearance and Isaac's mascot status, I will to bet on Isaac as the franchises playable character unless a fourth Golden Sun game featuring Matthew is released between now and Smash 4. Isaac has mediocre/average support in Japan compared to the fairly large support in the West. However, Sakurai is obviously aware of the popularity and uniqueness of Isaac since he not included Isaac as an AT but incorporated his Move technique as his attack. Furthermore. Sakurai is aware that Move does no damage in the Golden Sun series and applied that into Smash. Thus, Sakurai did his homework like a good little boy.
Isaac has fairly good chance to become a playable character and a 50/50 chance of representing Golden Sun at this point in time. What happens between now and Smash 4 will determine if Golden Sun would be represented and who will: Isaac or Matthew.
For Isaac:
Want: 4
Likeliness: 3
Starfy Prediction: 3.50
Nominations:
Skull Kid x2
Saki x2
Shulk x1
All S's.