It looks like a Link to the Snitch has resided, but if you do decide to come back...
A while ago, Sakurai decided who would be in Brawl as a playable character, and who wouldn't. Tell me, do you think he thought to himself "Well, I'd love to include this character Isaac as a playbale character, but Nintendo didn't make him, and that makes him third party and no/someone took that spot already, I guess he won't be in." If Sakurai really thought he deserved the spot, he wouldn't look at who made the character, he would look at who owns him - Nintendo, and there is absolutely nothing you or anyone can say about this fact, because it's right there, in the credits of Golden Sun. But of course, you knew this already right? I believe you actually like the character, and yet you shun his inclusion in this game? Odd...
Sakurai already acknowledges the fact that Golden Sun exists, whether that idea comes from the fact that he put both Isaac and Felix in his polls, or the fact that he READ two people telling him Isaac would be a really good choice for Brawl, or reading one from Felix. It could come from the fact that he LISTED and HIRED a composer from the game(s), or even the fact that he read himself that Golden Sun was one of the games that people had requested music from to be in Brawl. Sakurai knows this series exists, he may not know how popular it is, but he certainly knows it deserves some representation in Brawl, he just wants to see if his game's fans think the same.
Why would Sakurai add Isaac then? Because it's just as easy to add him as any first party character, that's why! You know why? Because he IS first party! If you made a time machine, and I bought it from you, does it matter that you made it now? Maybe to you it does, but not to the rest of the world.
If you are going to come here and complain about Isaac's chances, there are very limited things you can say, and I'm afraid, your entire argument means nothing. The only piece of proof you can actually state that says that Isaac won't be in as a playable character is his popularity. Notice, give this board a character list of 40+ GOOD characters, have Isaac included, and only a handful of people will remove him. Why do you think that's the case? Because people are INDIFFERENT to his inclusion. No way huh? You would have never guessed. Isaac is one of the 'safer' characters for Sakurai to add, because of that same reason, he won't loose fans from adding him, only gain Golden Sun fans, and add many fans to the Golden Sun series.
Arrg! I feel as if writing this was a waste of time, as you will most likely listen to nothing I say, just as I acknowledge what you say, but do not take it seriously because it conflicts with my (no, our) opinion(s). But I guess that's how humans are, eh?