There are a lot of little subtleties to this game, but I'm going to focus on one idea (broadly,) so here goes...
You earn experience points from the battles (not sure about treasure) and you need those to level up. To ascend (get new powers for) your adventurers, you need to collect various sets of "evos." (I believe this is shorthand for "evolution.")
The quantity and the type of "evos" that you need to ascend each hero varies by the hero.
I would imagine that some are common, some are rare and that after each ascension, the quantity and rarity of the evos for the next ascension will be more difficult to obtain. See below --
What if rpg characters had to earn EVOs and XP to level up?
XP would progress in the normal fashion.
For each level, or milestone level, the character would need to achieve certain objectives (Touchstones in my model) IN ADDITION TO gaining the requisite amount of XP before they can level up.
Here's an example --
|3||4,000|| Max Damage with Melee Weapon;  Critical Hit with Melee Weapon|
|5||16,000|| Defeat 4+ HD Monster in single combat|
|7||64,000|| Participate in Warrior's Tournament;  Defeat negative energy undead creature;  Fight a Giant|
|9||256,000|| Defeat 7+ HD Monster in single combat;  Fight creature with breath weapon|
|11||450,000|| Defeat a 9+ HD dragon;  Defeat a spellcaster|
|13||650,000|| Defeat 10+ HD giant in single combat|
Another idea would be to have players roll on a chart after every relevant encounter (relevant to each character class) and if the dice are kind, they would earn a Touchstone for their character. Perhaps different types and quantities of Touchstones would be necessary to advance to each level.
More on that later (when I get a chance to work up the chart...)