1. Ability scores generation method?
2. How are death and dying handled?
3. What about raising the dead?
4. How are replacement PCs handled?
5. Initiative: individual, group, or something else?
6. Are there critical hits and fumbles? How do they work?
7. Do I get any benefits for wearing a helmet?
8. Can I hurt my friends if I fire into melee or do something similarly silly?
9. Will we need to run from some encounters, or will we be able to kill everything?
10. Level-draining monsters: yes or no?
11. Are there going to be cases where a failed save results in PC death?
12. How strictly are encumbrance & resources tracked?
13. What’s required when my PC gains a level? Training? Do I get new spells automatically? Can it happen in the middle of an adventure, or do I have to wait for down time?
14. What do I get experience for?
15. How are traps located? Description, dice rolling, or some combination?
16. Are retainers encouraged and how does morale work?
17. How do I identify magic items?
18. Can I buy magic items? Oh, come on: how about just potions?
19. Can I create magic items? When and how?
20. What about splitting the party?
Just some quick thoughts for post apocalyptic settings.
Question #3 is iffy, depending on the pre-apocalypse tech you want to use, or whether or not you're including magic in the setting.
Question #10 should probably switched to something about gaining/losing mutations.
Question #17 should be replaced with a question about figuring out pre-apocalypse tech.
Question #18 should replace magic items with artifacts/pre-apocalypse tech/energy weapons. Potions should probably be replaced with either vehicle fuel or some other energy resource (i.e. charging power sources or buying power sources).
Question #19 really depends on the pre-apocalypse tech of the campaign.