I'm honestly struggling to think of a single game that legitimately "gets better" after a really slow, or just outright boring or bad first few hours. Most of the time I end up trying to imagine how the game could likely improve with the current systems at hand, and keep waiting for that wishful thinking to actually materialize, but it rarely ever does and I end up quitting halfway through the game (see: every Ubisoft game ever that's not the first and second FarCry or Rayman). The thing with Final Fantasy XIII too was that it didn't really become "better", it just became more open. The characters and gameplay still remained trash though.
Kingdom Hearts 2 FM?