>I doubt there's anything inherently wrong with the quarian immune
>system at the species level - evolution doesn't work that fast. It's
>an environmental syndrome, and it doesn't apply in Valhalla.
The fact that, canonically speaking, quarians can link their suits together and adapt to one another after a period of brief illness makes it pretty definitive that this is the case as well.
The quarians could actually probably walk back to not needing their environmental suits aboard quarian ships; it likely wouldn't be super-hard to do from a purely technical standpoint. But there would be some guaranteed percentage of deaths involved and it would be a very time and resource intensive undertaking, one that they'd then have to repeat if and when they retook Rannoch and had access to the pathogens and parasites they evolved symbiotically with.
Within the context of UF, assuming something doesn't happen to change the status quo ante (such as peaceable contact made with the Geth Consensus or Tali'Shukra coming back from her survey mission being all "Hey, uh, guys? We're the proud owners of an unspeakably ancient genocide machine that we've mistaken for a habitation ring. Go us?") I imagine some conflict eventually arising between quarians who are all 'fuck yeah, lets settle on Halo and stop living like lab specimens' and Rael'Zorah-esque hardliners who declare "Rannoch or Bust."
>Out-of-band, it's because her blindness is such an integral part of
>her character design that she's kind of not Toph without it.
There are ways you can edge around that, but they mostly involve doing some AU contortions that end up being the entire point of the story, which is something UF has done before but would certainly not be called for in this case.
I read a pretty decent Steampunk ATLA AU once where Toph had replacement cybernetic eyes made of metal that she should use her metalbending to pop out of her head and fly around and spy on people with. It was sort of neat.