COULD HAVING ALL HP/MP GROWTH TIED TO CONSUMABLE ITEMS WORK?

Dark Cloud did this for HP and Defense (also the thirst meter, which together were the only stats not dependent on your weapon.)

Since new treasures would appear as you built up the towns in the game, it gave a nice sense of anticipation when you were searching around the places you'd restored. But if you're going to go that route, don't force the player to scrounge around too much for the stuff. Having to check every damn nook and cranny while playing a game is just a recipe for neurosis.