Let heroes start out wimpy, require some grooming but w dedication become major forces.
Exactly.
At low levels heroes should need to be cautious and have other troops supporting them (although fighting class heroes should still be able to survive on the front lines in a fair fight, just not when outnumbered/outleveled).
Mid game heroes (eg level 10) should be pretty strong but still quite vulnerable if they get themselves into a bad situation.
Late game heroes (eg level 20) should be very strong, 'legendary' one could almost say. They should be killable by other 'normal' troops but only when outnumbered against strong 'normal' troops with good traits, weapons, etc.
Currently mages sort of fit this, a high level mage with sufficient mana can own. However the three fighting classes start off weak and get steadily weaker as the game goes on.