I am very sure that I can assume that all of you, just like me, want much much more Golden Sun. I have a theory that I believe to be very good, although not infallible. Not only that it proposes a reasonable way to have many more installments in this series. Anyway onto my theory.
First we have to deal with game number four. This is the easiest one to work out and has the most support for it. I believe that just as Issac was guarding the area of greatest light as the Sol Sanctum, Felix will be guarding the opposite, the place of greatest darkness which I think to be Anemos Inner Sanctum. This is due to the fact of Dullahan being housed here for the first encounter with him. As of right now, Dullahan is the closest we have to ultimate evil. The Proxian antagonists from the first two games only wanted to save their people. Tuaparang is merely a power hungry nation with an inclination toward bad deeds to get that power. Alex is the same way to a bigger extreme. Dullahan on the other hand, claims to guard light from being used. That is an act of evil and darkness. I also believe that they will not have shared their theory with anybody else barring possibly Kraden. Issac would have told Garet that they were studying aftereffects of the Golden Sun. Felix will be with Sheba and will have told her that they were looking for her ancestry. I think Sheba as they seem to have a close friendship, and she is the only other original eight adept not accounted for if I am not much mistaken (then again, perhaps I could be mistaken.... it has happened before). Both of their children (not going to comment on potential marital status), and possibly Ahri (or whatever the wind adept in Anemos was called) will travel to solve some problem that will occur while they are there. Along the way their party will grow to eight, they will join with Piers who will take them on his boat, and they will be plagued by the two other generals from Tuaparang. All of these events will take place along side the events in Dark Dawn. At the end of the game Felix will decide they need to check up on Issac and they will arrive at the Sol Sanctum mere moments after Matthew, Karis, and Tyrell.
In game five, Felix, Sheba, and Piers will rush off to the Sol Sanctum, telling the children to find the other three Warriors of Vale, and the five other characters from Dark Dawn. Rief and Kraden will join the group almost immediately, and they will have to relocate the other adepts, while battling all four Tuaparang generals. I really haven't worked much else out for this game, and some side quests and such will need to be added in order to make it longer and more fleshed out.
Onward to game six. In this game you will proceed back into the Sol Sanctum and once inside you will find that all this time there has been an underground civilization. You will find the adepts that proceeded you fairly quickly and then help out the city that has taken you in. All to soon you will find that they worship Dullahan, or some other ultimate evil. They are also the cause of the psynergy vortexes and have been siphoning power for themselves. You must defeat Dullahan as a storyline boss, destroy the way to make psynergy vortexes, and topple the civilization to prevent future darkness.
Finally game seven, the last game in my theory. You will return to the surface to find that time moved differently underground and that about 100 years have passed. In this time Tuaparang has taken over all of Weyard. In this game you must find a way to face off against the leader of Tuaparang and defeat him. After you do so, he tells you about how Alex had been pulling the strings from the beginning (just like we all thought) and has outsmarted us all. He was expecting Issac to get part of the Golden Sun and used it to make Issac cocky, then took it without Issac knowing. Then, when Dullahan was defeated in the last game, Alex confronted him and took all of Dullahan's power as well. After gaining Dullahan's power, he is going to go defeat the Wise One and take his power as well. You must race off to stop him, and arrive just in time to watch the Wise One get defeated. You get the rest... beat Alex in a crazy fight that takes forever and makes Dullahan look like a sissy. Yeah, fun stuff.
Now for some general game-play stuff. First a new way to have different adepts in battle will need to be implemented. If you can have four of any one type of adept against a boss that is weak to them, it would trivialize their difficulty. The level cap would need to be drastically raised by the time the seventh game roles out. Also another thing would be raising the number of djinn, and making new and more powerful classes available.