I don't think they are gonna kill of the series. Camelot always wanted to make a Golden Sun game, but priorities required them to focus on the more notable Mario Golf and Tennis games.
They aren't gonna end the series with just a shot of the Mourning Moon (which I bet some players forgot about), but I think they should've had a less obvious cliffhanger due to the fact that it was probably going to be a while before a fourth installment would be released.
Now, I'm glad that this wasn't Assist Trophy week, and I'll explain; I know that I want to get the information out as quickly as possible because I'm not into waiting over something that makes me sweat a bit, but if we ended up like that, it would've been a shooting gallery. I also feel as if that if Isaac made an appearance outside a reveal, he wouldn't be a playable. It's for those reasons why I think there's a possibility Mr. Game & Watch might not be playable this time around since he was featured at the end of the Pac-Man trailer and his profile didn't appear on the website (and I know Toon Link was featured in the Spirit Train stage at the debut video, but he is merely a decoration in that stage if he's not playable).
Also, with an earlier post about the difference between Golden Sun's involvement in Brawl compared to Punch Out!!'s, somebody mentioned Gyromite. Now, R.O.B. was a playable character almost entirely because of the Subspace Emissary storyline due to how the R.O.B.s were being used by Ganondorf and the Subspace Army and that the Ancient Minister was the playable R.O.B. who decided to fight back in an (unfortunately) hopeless effort to save its kind. Since Sakurai has stressed that the "adventure mode" in this version (at least the 3DS) is merely a giant map where you beat up a bunch of Nintendo enemies and there is no semblance of a story, we won't get those kinds of characters.
Ok riddle me this, if they're making these Mario Tennis games, why not include Golden Sun characters in them?
Square included Final Fantasy characters in Mario Sports mix, so don't you dare say it would be weird to have the adepts play tennis if we already seen two mages, and a ninja play Basketball, Hockey and get this they were playing Volleyball and Dodgeball.
Now it definitely doesn't sound too weird to have Golden Sun characters in Mario Tennis, they should of had them dlc instead of stupid Yoshis and Metal Mario, that was so stupid, I still punish Yoshi by beating up on him for ruining Open's QR potential, that stupid loser dinosaur horse thing.
As for Golf, I don't care about that series, its Golf, I hate Golf, unless they turn into something where you hit everybody with Golf clubs in a no holds barred hardcore match, forget putting, just hit each other with Golf clubs.
Btw I don't think I'm going to want to play Golden Sun 4 if Isaac or any Golden Sun character isn't playable in this game, it would be it for me, I'd quit on the series, Golden Sun has been requested for Super Smash Bros for over a decade and all we got in Brawl was a stupid trophy, if we don't get that Golden Sun character now, that's it, I'm probably going to be done wit the series, I would no longer have the will to continue playing Golden Sun, the series isn't getting the proper representation, I command some respect for the Golden Sun series, and if it doesn't get the representation I want it to get, which is seeing a Golden Sun game more often than we are seeing them, including Golden Sun characters in games Camelot is making like Mario Tennis, and of course playable characters in Super Smash Bros, if these demands aren't met I'm leaving Weyard, or in other words the Golden Sun series as a former fan, I don't find this series fun because of the mistreatment its getting.
I wouldn't be surprised if Isaac was a hidden character not announced before release if he's in, and hopefully he is a playable character.