I don't think player character abilities should be changed alot. Some epic abilities are way stronger than others, however. Balancing should come from clever changes to monsters. With changes like this both monster damage and hp could be lowered and game could still offer challenge. I will cover some areas of gameplay that I think have made the metagame what it is today.
Monsters not using things that bypass parts of layered defenses.
Displaced shadow fading monks with maxed dodge and some PRR are so popular because every one of those abilities works most of the time on physical damage. Each of these should have their uses but they should not work always.
- Why are there no monster archers with force arrow imbue? Players can get it at level 2 at earliest yet the most epic of archer monsters dont have it. It should be common ability on archer monsters. This ability bypasses incorporeal. Some melee monsters could come with ghost touch if it makes any sense lore wise.
- True seeing can be cast at level 9 but you rarely encounter it on monsters. If the organization you're fighting against has clerics or mages there should be a good chance of monsters having true seeing. This would also affect invis runs though. Not sure if it is a bad thing.
- It's ok that PRR always works. Problem is that you can get enough PRR from enhancements, items and feats that you get past optimal investment point making extra PRR from armor rather meaningless. There could be adjustments to PRR mechanic to make armor count more. One option is to adjust the 0.65 in PRR formula up or down depending on armor worn. So for different armors same PRR yields different results.
- There are also some abilities that bypass a portion of dodge rating. Certain monster types could use these.
Epic ward.
This ability simply ruins some playstyles that are viable in heroics. Rogue with crippling strike. Ravager with Mutilate and Cruel cut. Cloudkill using archmage. Bard with Suggestion. Even some Epic Destiny abilities suck when rats, wolves, bears, spiders and other epic monsters are immune or highly resistant to abilities you might have even specialized in.
- Give dispells to hit/AC treatment.
- Replace epic ward with buffs or make it dispellable. Make different 'epic wards' suitable for different monster types. Normal animals or vermin should not gain any kind of ward while strong casters could have very strong magical protections. Melees could come with package that improves their performance(true seeing, haste, blur, freedom of movement, death ward, immunity to mind spells, abilities from old epic ward etc). All these should be dispellable.
- Most casters and potential UMD using monsters should come with night shield or shield. It is level 1 spell afterall
- Giving dispell ability to some classes could also buff them if dispell is useful. Paladin Holy Sword could become more like Holy Avenger as self only weapon buff with dispell attempt on crit or so besides other benefits. Barbarian Occult Slayer could also gain dispell attempt ability at tier 5 or so.
- This could be a good time to give abjuration spell focuses an increase in dispell chance. Maybe also for abjuration archmages. Wider variety of build choices would be nice. Currently no one even thinks about focusing in abjuration.
Cost of getting a mob dispelled would play a big role in caster balance and should be adjusted carefully. Buffs on monsters can hurt both melees/ranged and casters depending on buff choices. Some monsters could even get buffs specially designed for them. For example now very few enemies use fire shield style abilities.(Melee nerfs are not needed at the moment. Yet it would be a buff for those melee builds that can dispell when compared to ones that can't.) Also Spell mantle/globe of invulnerability is rare these days.
Kiting.
This is last because I'm not sure if much/any changes are needed. This is atleast partially just a symptom of way things are now, like Fhauvial said. Most monsters lack reasonable countermeasures against kiting players. This makes kiting much less risky than using melee. Kiting should still be viable but atleast require more tactical choices. It should be dangerous to kite entire encounters.
- Have AI recognize kiting behavior(damage dealt vs. hit attempts taken. Monster attempting to attack in melee for time period but unable to do so or something like that. Probably not easy to come up with criteria that won't also consider CC casters or others as kiters. It might not be that bad even then though.)
- Archer mobs could target kiter so atleast some damage is caused. Archers could also use special attacks like leg shot or some ability that stacks per hit basis so the slowdown isnt as instant and there's time to react. This should help melee monsters to land more hits on kiters.
- Melees could use more tactical attacks on kiters, or switch to side abilities like shadow dagger, binding shadows or other debuffs suitable for particular monster that could help them catch you.
- Also casters could spare their Power Words for when kiting is detected
AI changes are probably not easy at all to implement. It could also be easy to overdo this. Most archers could just gain ability on their attacks that stacks slow effect on players. Nothing huge probably. Just 1% per hit could be enough.
These are some ideas that I got that might make any sense. If you like any of them please comment them. It took a while to come up with all of these. Some of these might have results I didn't think about. Most importantly these are ways to make monster hit points and damage more reasonable while still keeping challenge and hopefully balancing different playstyles in process.