I don't know if this was already mentioned. I'd like to have some sort of warning when health reaches a critical point, maybe 10%. This could be a special sound or some UI blinking.
I'd boost that up to 30% or so. At 10% there's barely anything you can survive unless you're visiting low tier planet in comparison to your armour.
Make it an option with a slider, then you can choose 1-100% to your hearts content. Health Alert: 0--------------][-----------100
I can already see people modding it so you get HEV suit's comments when receiving huge damage instead of some random blips. "Major fraction detected, blah blah blah".