Regarding skill ranks and unlocking, would you consider a nonlinear tree-like unlocking system? This way players who like a particular aspect of the game don't necessarily need to slog through things less enjoyable to them.
For example: (Assuming vanilla up to HMP starts unlocked)
Beating HMP vanilla unlocks UV vanilla, which when beaten unlocks N! vanilla
Beating HNTR (or harder) vanilla unlocks Angel of Berserk, Marksmanship, Darkness, Light Travel, Haste, and most others (on HNTR difficulty)
Beating Light Travel (any difficulty) unlocks HNTR Purity and Impatience
Beating both Impatience and Purity (any difficulty) unlocks HNTR Masochism
etc
Beating any challenge on a certain difficulty unlocks the next one (e.g. beating AoMr on HNTR unlocks HMP AoMr)
I don't focus too much on xp ranks as they don't actually affect how the game plays, but with that said I don't mind the system in place now, with less inflated numbers.
One possible alternative is to have (material) (monster) ranks, where you start at a less-prestigious material, and go up the monster ranks the same way it's done now, with much lower numbers. Then, once Cyberdemon is reached, raise the numbers, and move onto the next material.
For example: (each rank requires fulfilling all the requirements of all previous ranks)
Bronze Former Human
- kill 50 former humans
Bronze Imp
- kill 50 imps
- kill 10 enemies melee
...
Bronze Cyberdemon
- kill 3 Cyberdemons
Silver Former Human
- kill 200 former humans
and so on.
So a player cycles through all of the monster ranks at lower quantities, then advances to a more precious metal/stone/whatever, then cycles through all the monsters again at higher quantities.