Evolving heroes? How to keep a map interesting after max level?

Alright, so I have a big ambitious map, a defense type map, with heroes able to go up to level 25. Problem is, they're achieving that level more or less by a little over half of the total waves I would ideally like to have when the map is done, not to mention side content, etc. On top of that, presently they run out of skill improvements after level 16, barring my increasing abilities higher from 4 ranks, and ults from 3.

I want to keep the map going considerably longer, and still keep it challenging and interesting. The best idea I've heard to accomplish this thusfar is to have an 'evolution' for each hero that changes them into a 'late game' version of themselves, with fresh abilities and level 1, yet capabilities scaled to enemies of the new difficulty there.



I'm stumped, though. The balancing questions this brings up make me a little cross-eyed in frustration, and with the number of heroes in, I know I'll be hard-pressed to come up with more creative abilities for them. As it stands, I am trying like hell to avoid just slapping generic boring "Oh, this hero has critical strike. Yay." kind of abilities on people. I want the heroes to remain interesting. What can I do? Any suggestions?

Or you can just change the exp gain rate.

Go to Gameplay Constants and then down to Normal XP Gain or something, there are a buncha variables there that determine the exp given to heros. Also, make sure to bring the lvl down of the waves of creeps for reduced exp.

item, quest, stat, trigger-based masteries (something like Vampiric touch - gain 0.02 damage bonus for each kill) and all these kinda things