Long story short, my players' characters (who are still mostly in denial about their force abilities) have "accidentaly" found the hyperspace route to the ancient jedi world of Tython, and are rapidly heading there, having no idea what's in store.

The wookiepedia article does give me some ideas about what to do with Tython, of course, but I'm looking for more. I'm all ears for any cool ideas you can come up with! Especially, though, I wonder:

the wookiepedia article repeatedly has some lore about Tython being dangerous for non-force-sensitives, but never explains exactly how. Now, I don't care about canon, so I'm fine with coming up with my own solutions, but I do wanna use this theme somehow, because there's several non-force-sensitive NPCs in the group that I could threaten. Could it have something to do with the "force storms" mentioned on wookiepedia (who, again, never really explains what that is)? Or maybe Tython is somehow perilous to navigate if you're not sensitive to certain force visions that will lead your way or something? Or maybe there's a strange mist that separates the party and only the force sensitives can find their way out? Give me anything you can come up with that's interesting!

what about some sort of infection or mind twist that being Force sensitive is a barrier against.

Nexus of Power discusses Tython, as well as discussing the effects of Force nexuses on living beings. Mechanically, it would be upgraded checks, increased difficulties, and boost and/or setback dice. Fear checks are a good place to start, illusions, hallucinations, disorientation, and so forth.

How about a native predator species that is attracted to/hunts non-force sensitives?

Tython experiences extreme weather that's driven by the force. I believe there are some Dark Horse comics on marvel unlimited that deal with Tython called Dawn of the Jedi. There's also a novel by the same name if you're a fast reader.