Will
Remove the current Will save.
Replace with an attribute based on Charisma. It works like the Contest of Wills, in that you resolve battles of will with a bidding war.
It can be regenerated through certain morale-building actions, and can be sapped directly with certain spells.
When you run out, you can get more by suffering Strain.
Different Saves
Remove Fort/Will.
Instead, require a "Strength save" or "Charisma save" or whatever. You basically check your attribute, and, if you fail, you suffer damage to said stat.
You can auto-pass by suffering a bunch of damage to the stat.
Incents PCs only to save against the things they really want to break out of.