I primarily come from Sony games such as EQ2. The crowd seems very different here, so I'm fully expecting to be laughed at for this. That's okay. I also acknowledge that there are a lot of more important things to be looked at first.
Nonetheless, here's my suggestion. One of my favourite things in EQ2 was house and guildhall interior design, and it's certainly one of the two things most likely to tug me back. Masses of house items are available in game whether as player-crafted furniture, quest rewards, mementoes from important in-game events, or simply purchased from the 'station cash' marketplace. It allows us to create unique and creative homes for each of our main characters should we wish.
While it took years for EQ2 to build up the house item catalogue it has today, I would love it if SWTOR could introduce something like this in a much smaller way -- the ability to customise ship interiors. I'm talking trophies from important kills, being able to put something pretty in girlie companions' cabins, the odd 'wanted' poster for the walls - that sort of thing. The bare utility of the ships isn't realistic when they have so many people living on them. Let us be able to put a personal stamp on them should we want.
I love this game. Not having the ability to personalise living space is very far from a deal-breaker for me. It's just... it would be nice. ;