My top 5 choices:
Having a unique story for each character was a really cool part of this game, and while I haven't finished all 8, expanding on them is something needed every once and a while. I'd be fine with them giving about 2 quests or something per each new planet something like -2 quests on Ilum, Black Hole, Section X, Makeb, CZ-198, Oricon, and then some final encounter wherever.
Companions, and companion stories, were a really cool thing to have. There is definitely more room to give another 5 or 10k affection, and to give companion missions to everyone rather than just the first one. I would prefer expanding on current companion than adding a bunch of new ones.. Also, I feel like companions could be a lot more useful in something like an operation, or special PvP arenas with companions - It's nice that they are adding them into the Galactic Starfighter though.
They could really add a lot to the existing planets, and they could make really cool new planets. Adding some separate area to the lvl.1-10 worlds would be really cool, like Section X and Black Hole. These sections could be placed at night time for extra awesomeness, and opposite factions could be able to go to the endgame areas of them. Adding underwater content is probably not something that would happen, but an underwater planet would be quite cool.
Operations are always cool, and now that the Dread Master storyline is done with, a new villain is needed, giving the opportunity for different types of boss fights. I've always wanted to see a turret-orientated fight like Kaon Under Seige, or a fight where are a slight or big help. A boss fight where some NPC-starfighters could help would be amazing.
New customization is needed for all species, and I don't see why they couldn't have added longer hair or different colored hair to each species. Body Tattoos would be an awesome way to show off the more skimpy gear, and as a lot of people have said, more male skimpy gear is wanted.
- A 6th one because why not:
Minigames have been requested ever since launch, Pazaak and Pod-Racing have been the most suggested, and they could make revisiting worlds like Nar Shaddaa and Tatooine more worth it. I could see a Pazaak floor being added to the Fleet as well..
I hope this thread is looked at to see what players want, and this is really nice to have been put together, OP.