I am suprised that noone has made this topic yet!
It would be really cool to have a blackout affect when above 20,000 in a GA aircraft, and as you climb you start to lose focus and stuff.
5,000-8,000ft (may get a headache but still fine)
8,000-12,000ft (probably gonna get a headache and may be short of breath)
12,000-14,000ft (liveable but may lose some minor focus blacking out becomes a risk)
14,000-18,000ft (blacking out becomes a major risk and fluid may build in the lungs and brain)
18,000-24,000ft (definitely will black out and hypoxia starts and the pilot may die within minutes)
ABOVE 24,000ft (hypoxia and Death)
This would be very realistic and cool. Also you could choose where the pilot lives so that he could already have a headstart for altitude.