Sulfur has also become a bit more difficult to avoid due to currents, so it would also be nice if very small amounts don’t trigger damage at all.
Oxygen and UV tolerance still have the same problem as I also saw pointed out on the community discord recently: with it only being an MP-sink there’s usually no reason to not pre-evolve them before you need them. That’s also why oxygen tolerance on the bioluminescent vacuole does not make it useful. But changing that would also be a big task, and I believe some previous suggestions for ongoing costs were not that well-liked.
Limiting how far you can push the UV resistance slider is in my opinion possible and potentially fun, but would require a prokaryotic equivalent to the melanosome.