Not sure if any of these have been brought up, but I feel we're sorely lacking with the off-duty outfits we have in the game at the moment. All the real good outfits that would fit this category have been in the earlier movies - particularly the first four. So, my suggestions:
1)The outfit worn by Kirk, the red outfit with the white shirt with the wavy flap thingy, in The Search for Spock and The Voyage Home (including an option to include the leather jacket from TSFS).
2)The outfit worn by Dr. McCoy in those same movies.
3)The white robe (with optional headband!) worn by Spock in TVH. Wouldn't make sense for non-Vulcans, but what the hell.
4)The darker meditation robes (again, might not make sense for non-Vulcans) worn by Spock in The Wrath of Khan, TSFS (it was considered his burial robes, but eh) and The Undiscovered Country.
5)The scientists' outfits seen in TWOK and (briefly) TVH.
6)The 70s-type outfit worn by Dr. McCoy (yes, including the necklace - beard optional) when he beamed onto the Enterprise in The Motion Picture. Just for giggles.