That would bring more fun to the game, especially for high-level players who can't fight any more challenging mobs.
As you now, mobs drop a set amount of xp right now. We should keep that number as minimum xp drop. Upon every level-up, the hp, ac, bc and ad of the npc should increase by 10% of the base stats. To make that happen, the player's stats (hp, ac, ad, bc) would need to be calculated in xp.
But that should only trigger under particular conditions:
-The player's xp (p) is 2 times greater than the monster's xp (m) to prevent overpowered mobs:
p/m= >2
For example, m is 400 and p is 1000: the stats increase by 10%.
If m is 1000 and p is 500 or 1500, the monster won't level up.
-There should be a max amount of monster level-ups: 20 times
If the player doesn't start with that feature because of an old version, the monster would need to level up upon loading the save until the p/m would equals 2 or the max level 20 is reached or the player's level-ups are depleted.
I also suggest adding an option to the game allowing the player to set the difficulty of the monsters. Options are: very easy, easy, normal, hard, very hard and impossible like in normal game. If the player changes that option, the stats need of course to be recalculated:
Very easy: mobs drop the min amount of xp and stats aren't increased when player's level is changing.
Easy: m*1.05; mobs level up slower; p/m >3
Normal: see above
Hard: m*1.15; p/m >1.5
Very hard: m*1.2 ; p/m >1.75
Impossible: 1.25; p/m >1 I think, that'll be but a nice challenge for hardcore players

I know, that's more coding but I definitely think, it will make AT much more fun for everyone.
What's your opinion?