I started my now-haitus Perseus ship story with ensigns. My character was a Lt. Cmdr (the CMO), but as I was narrating, and no one wanted to play a medical officer, it worked out fine.
They were: A pilot, a science officer, a security officer, an engineer and an ops officer and most of them worked the night shift.
Over the course of the story, the Perseus took a major hit, but was able to be repaired. However, the ship lost their Ops officer, their XO, the pilot turned out to be a Cardassian deep cover agent and killed the Chief Science Officer on the way out, and the Chief Engineer - and half of engineering - died in a plasma explosion...
So, pretty soon those ensigns were rising through the ladder of responsibilities, one or two were the second-in-command of those departments. As time went by, most have since reached Lt. JG or full Lt, and are now mostly the heads of their departments. It was a long, slow, climb, but they loved it.
The Doc
So you think, 'Might as well,
Dance a Tango to Hell,
at least I'll have Tangoed at all.'
-- "Rent," Jonathan Larson