Sometimes it is a pain to have to run around the wild looking for bad guys to battle so I can level up my heroes.  Being able to research and build facilities to train heroes and level them up.  Make them hard to get and expensive to use, but with something like a school of magic or ivory tower one could level up a unit in intelligence for magic casting.  One could build a gym or training facility of some sort to improve defense and constitution, things like that.  You could pay out the nose to have a percent chance that certain stats will be boosted.  Something like what they had in the old game Digimon World 3.  You earned training points by leveling up, then you could spend those at one of the gyms in the game.  You could train in just about every stat.  You also had the choice of spending 1 or 2 tp, or was it 3... no matter, you could spend more and have better chance to gain more stats.  1 TP got you 3 pass/fail chances, and if you got all three you got a bonus.  2TP got you like 5 with a bonus if you passed them all.  Now, I'm not saying use that, but that's what comes to mind.  Plus, I like to keep certain heroes at home to use their abilities, like farmer.  It would be nice if they were not always fated to be level 1 for the rest of the game... well, assuming they don't survive a few raids on their town.

Anyway, just a thought.