Probably been suggested before a million times, but it would be extremely cool to have a holodeck we can access in the ship interior for practicing combat mechanics and tactics. It would be a nice way to include some simple "tutorial" content beyond the intro scenaries, and would fit perfectly with the "real" use of the holodeck in the shows. There would be no rewards or SP for holodeck exercises, and all the maps would be tiny and simple (like a single room or small clearing).
Make it so you can beam into a simple indoor or outdoor map, with or without your away team, and face either "passive" targets that just stand there and die while you wail on them, or "active" ones that try to fight back. And you can choose between one and five enemies with stats matching your current level and difficulty slider. When all the mobs die, you get a simple prompt to respawn them, and you can "beam out" of the holodeck at any time.
A space mode which puts you into a small encounter system with similar options would also be nice. If players could do mini Foundry encounters for "Holodeck Training" missions, I could even see this being a popular format for UGC.