Kingdom Hearts
I put a ton of engines, extra wings, and laser cannons on my gummi ship. It is awesome. Building a cool ship is kind of expensive, but now that I can cure myself with magic (and regain MP just by hitting things), buying parts is the only thing I even spend Munny on. I know for a fact that you don't even have to build onto your ship to complete the game, but I enjoy it a lot.
The combat keeps getting deeper: new magic spells, new abilities, new enemies and scenarios. There are even new weapons, although not very many. And what really keeps things fresh are the plentiful world locations. The Disney franchising made the game seem very weird at first, but the numerous atmospheres are really great at maintaining variety throughout the gameplay hours. Even the "plot" is becoming more interesting.
Progress: Atlantica