We've talked about the environment and the physics of this place. We know that Nomolas is ineffable, that he's not going to be the source of conflict for the story. We need to decide what the major story arc of the campaign is going to be, so that we remember what we're supposed to be dealing with, and not get sidetracked by other concerns.
We agree that Mehir is going to go mad, and that his madness will represent the plight of the Summoners in general. Perhaps other Summoners have already gone mad and now it's Mehir's turn to face the problem.
Whether Mehir experiences isolated madness, group madness, or societal madness is important.
Isolated madness might be like Tom Hanks in
Cast Away. "Chuck" crashes a FedEx plane on a deserted island and is the only survivor. He faces survival problems and tries to escape, but it's a hugely daunting problem that takes a long time to solve, and the loneliness tests his sanity. He partially deals with it by inventing a friend, a volleyball named "Wilson." He nearly commits suicide but instead takes his chances with a raft. He loses Wilson while making his escape and it's emotionally devastating to him, like he's lost a real friend. When Chuck makes it back to civilization it's very difficult for him to re-integrate, because his wife has remarried and so forth.
For group madness, compare many sci-fi episodes, too many to count, although as usual I'll point out a STTOS episode,
The Naked Time. Oddly enough you can watch any STTOS episode in full nowadays, either on the
Star Trek website or the
CBS website if some episode is missing. So if you're too young to know what I'm talking about, you can rectify that.

Hmm they've got ads, so they're not doing it out of the generosity of their hearts, or just to promote the modern Star Trek (2009) movie. Anyways, in this episode the crew contracts a virus that lowers their inhibitions. This gives all the actors a chance to act, to flesh out their characters. Plot-wise, the crew goes through the usual problem of not knowing what's going on. Their aberrant behavior puts the ship's orbit in dire peril and the clock starts ticking to their doom. Not everyone is affected yet, so the sane members of the crew try to cure the madness before everyone goes mad. The doctor succeeds and everyone is inoculated, although it's the usual nail-biter ending. I guess plot-wise that's pretty simple and not comparable to what we'd do. STTOS of course had to wrap everything up in ~50 minutes, so there's only so much to it.
Societal madness would be a society that has devolved into something psychologically unhealthy, for whatever historical reason. STTOS
The Return of the Archons is an example. A computer controls everyone's brain on the planet, leading to robotic, calm, emotionally blissful behavior most of the time, and wild violent orgiastic "Festivals" at periodic intervals. STTOS
For the World Is Hollow and I have Touched the Sky is another example. A civilization is living inside a hollow asteroid and doesn't know it. A computer prevents them from understanding the true nature of their environment, on pain of death. STTOS
The Way To Eden may also be relevant. Space hippies live in rebellion of Federation political priorities, and seek the Garden of Eden to live peaceful lives. Unfortunately their guru, despite a highly principled world view, really is crazy. Although the crew of the Enterprise assisted in finding the "Eden" planet, they hijack the Enterprise, steal a shuttle, and nearly kill everyone on board the Enterprise to make their escape. The "Eden" planet they land on is filled with acid grass and other violent botanicals. Most of the hippies die, in particular their leader, who is so determined to believe in Eden that he swallows poison fruit.
All of these "societal madness" stories have an underlying notion that "the leaders in charge are crazy."
Another major concern is
how many characters with some depth and character arcs will we have. In "To Lands Unknown" it's really only Rashti. Mehir gets promotions but his character doesn't change much in the course of it. When I first started contributing dialogue, I started to try to shoehorn a "more serious" take on Mehir's character, and an arc, but soon realized that the tone of the story was essentially comedic. It's also heavily weighted towards "introducing the world," like the Great Circle, the Abyss, the Dark Elves, etc. Mehir starts to go through a character conflict about betraying the Kharosians vs. reaching the Abyss, but it is not dealt with or resolved in the campaign. So Mehir as a complex character is pretty flat, although he is funny, and has a defined personality.
"To Lands Unknown" is primarily plot driven, not character driven. For Mehir to experience madness, we may have to deal with his character. It may be possible to deal with "madness" in more generic terms, but it may lack psychological punch.
Also, unless Mehir goes mad alone on a deserted island, other characters will be affected by his madness. This implies that they should have at least a minor amount of development, so that we can see the difference between what's sane and what's mad.