Imagine giving us a mission builder on the holodeck.
If they did that I would be a subscriber forever! It would also be amazing if players could own... a plot of land on any planet and customize it and build themselves a home, a farm, wine orchard(like Picard's). Let players have their own little zone/instance on a planet, in a hologram, in a city.
I'm in agreement with most people here in that captain's quarters would be very nice. I would also like to see:
Ready room (fully customizable)
Observation room (can be used for fleet meetings)
Main Engineering - These three could just be re-used from the tutorial
I would also love to see a runabout interior, as well as a shuttlebay where we could store our runabouts. I bought my runabout as a kind of captain's yacht, and I'd love to be able to access it from my ship. Maybe with a mechanic where when you launch in your runabout, you designate a space station, such as Starbase 1 or DS9, for your ship to go to. It'll be waiting there for you to pick it up when you're done with your joyride.