One thing about RPG's that I find interesting, the level up system. Getting stronger by doing battles. But, if you do too much, like grinding, or just getting unlucky with random encounters, your level can sometimes become too why, and parts of the game may become too easy. Which is where my idea comes in, unlocking new level caps. Here is an example.
At the start of a game, your level cap is 10. Pretty decent level for early game. After you defeat a certain boss, you get a message basically saying, "Level Cap Increased!" With it, you can level up to say, 20, giving you better stats, as well as new abilities you can get. That way, your never too powerful for what's ahead, making it so you can't just grind to win. You'll still have to think about how to win battles. If I had to compare it to anything, I'd say the Crystarium system from Final Fantasy XIII, just without branching paths and stuff.