first of all, I think DWC is a remarkable puzzle piece in the 3d printing world, which made having a dedicated Control unit on the Printer completely obsolete for me. My printer only has its power button and nothing else (it's sitting directly next to my PC though). Before I worked with Duet, I was constantly searching for "the perfect" UI, considering everything from 12864-style displays with encoder to full fledged touchscreens. So this shows how much of a game changer this well done web interface was for me personally.
That being said, this use case is not for everyone. To answer your question and offer my 2 cents:
I think DWC might have potential to be the truly universal UI for remote and non-remote operation of the printer. Therefore beyond the screen size adapting to smaller (smartphone) and bigger (desktop) screens, there should be different "modes" that you can set up. I'm thinking "printer display" mode which offers at least the functionality that classical 12864-style controllers (or Paneldue ) offer, and "admin" mode like the current DWC UI, offering full access to the printers configuration, files etc. "Printer Display" mode should adapt to smartphones or tablets, maybe offering more tactile UI elements (e.g. knobs for temperature adjustment, if possible), which leaves the user to choose if they want to use their smartphone, or fix a tablet or RPI display to the printer itself (offering the possibility to even add mouse and keyboard there).
I can't really offer any feedback on features I find missing in DWC, because for me personally there are none. Maybe one thing (which would apply to "printer display" and "admin" mode both):
My Anycubic Photon (DLP printer) uses a Chitu board and touchscreen, which shows a little representation of the model in the slicer. It's a screenshot of the slicer window, which makes it difficult to implement in our FDM world as I know of no slicer that offers this atm, but maybe there is a way to have a little graphical representation of the model in the Gcode file list?