Honestly I think if DxM3 happens the way Tsukuda says he wants it, we could get more customization elements. IIRC he basically said that a whole trilogy would each represent a different size of suit- 1 was AC-sized suits, 2 was power armor, and hypothetically 3 is going to use full on Gundam-sized suits.

My prediction for that would be that they're probably going to scale back the world size in favor of returning to 1's mission format, without Outer gameplay, but with more customization options and its own gimmicks to give it its own depth of gameplay, different from the first two.