I've been thinking that a possibility for a death penalty could be something along the lines of disabling perks for a turn or three. Some kind of effect that lasts over a turn but not too long, more than three turns would feel too long in my opinion. This wouldn't be crippling most likely but it would remove some pretty cool buffs and would be nice at representing a weakened state.
Or maybe you lose access to the mage class and perks you were using at the time of death for a turn or three. Something like that will discourage Poison Pill abuse without being too crippling.
I don't like the idea of that, strictly because if you died, you're clearly lacking in something. What will taking away mage classes and perks do, aside from make you lack in more things? This could lead to an unwinnable situation, and especially discourage you from trying new things. If there's some kind of penalty while you still have the power of the oblivion crystal, it needs to be very, very subtle. Maybe a chance of -1 morale so you don't just screw up constantly and deliberately. But I mean, it's way too easy and fast to die to cause actual major punishments to actually be good ideas. Even if it is just for a turn or two, you'll easily get to the point where you actually can't reach any wind generators anymore.
And for the record, once you don't have the oblivion crystal powers, you actually lose 100 mana when you do die. Every time. Pretty steep price in the endgame that you need to plan for. I don't think making it any worse is necessary.