Felix, as has been established, is about to plunge into the unexplored depths of Sol Sanctum in a search for ancient secrets that may save his world - or may, once again, doom it. But that's a concern for after they're found.
There's basically no way to run this the way his canon would; even if people with access to magic had signed up, the text-based nature of LJ, combined with my own limitations art-wise, make most of the types of puzzles Golden Sun uses unfeasible.
So what I'm thinking currently is
as follows: two of the bonus bosses from the second game, placed to make sure whoever comes to claim the secret is powerful enough to use it and hopefully experienced enough to be wise in using it; and from-my-own-head devices that scan the shape of the seekers' thoughts - not what they're thinking, exactly, but the pattern of the types of things they think, in order to make sure the secret isn't claimed by someone who's going to turn around and sell it.
The first of the bosses in question is Sentinel, a large, animated suit of armor. Sentinel's main trick is that he's completely immune to spells, apart from his own defense-boosting buffs. This will be of less concern with this group, I think, since neither of the volunteers has any spells to use. Sentinel is susceptible to the magical enhancements of weapons used against him, as well as to summoned elemental spirits. He's more susceptible to Earth, and less susceptible to Water.
The other is Dullahan, for which I apologize. Dullahan is also a suit of armor, but lacks a helmet. He has a number of unusual abilities, but the most devastating in canon are Djinn Storm, Fulminous Edge, and Charon. Djinn Storm is going to have no effect on most characters, but will be devastating for Felix; it converts all his Djinn to "Recovery" status, where they cannot contribute to his stats, give him access to higher-tier spells, or be used in summoning. Fulminous Edge is, essentially, a sword made of lightning, which causes immense damage. Charon is the second-most-powerful summon in the game, and in addition to causing large amounts of damage has a chance to automatically KO any of its targets. I will be toning this down, for the plot, so as to not guarantee a total party kill in the first few rounds.