I'll just leave the screen of DX50 as is. Bigger screen equals less battery life. Not to mention it would take up space which will be useful to add better components to improve audio without getting bulky like DX100. After all the main goal is sound quality in reasonable size and not visual quality.
The only thing I want to improve with the screen and UI in particular is better navigation. It would be awesome if they add support for touch gestures. They could display more info on the screen without increasing its physical size. Say track, album and artist navigation can be done directly on the home screen just by swiping up, down or horizontally.
They could also use hidden panels that could slide down or up to display functional buttons and could make them look big so you don't missed the target when you press on them. The home/back buttons at the top of the home in particular screen is easy to miss.
Same with the on screen play/pause, forward rewind buttons. Oh, the scroll bar could be thicker as well.