Here are some of my ideas:
Sell individual items on the cartel market because some players (myself included) don't like to gamble.
You could offer this as a subscriber perk.
Lower the prices in the cartel market so more people can buy stuff.
The costs of making a copy of a digital asset are negligible. So you can truly "make it up on volume."
Move the Command XP system to the Legacy XP system to prevent proliferation of game mechanics, lessen the grind across our legacy, and to save database space.
Allow f2p accounts to access raids and PvP so subscribers will have more people to play with.
One of the great things about MMOs is the network effect. More people will play (and pay) for your MMO if there are lots of other people to play with.
Offer a lifetime subscription because some players (myself included) don't like the monthly fee.
You could offer lifetime subscribers access to old content - The HK escape storyline for example.
Combine companions across Legacy so we don't have to grind influence on each character and to save database space.
Add more companion stories to flesh out companions a little more.
In addition to companion-specific stories, you could add a storyline where your romanced companion gets kidnapped. You can randomize the villain and reuse the story across all classes. Another storyline could be that one of your companions has been compromised (willingly or unwillingly) and is thwarting your efforts.
PvE Content for groups of groups so more people can play together.
Multiple groups can work together to recapture territory held by NPC enemies. You have multiple regions that people can travel between. Each region has multiple capture areas. Once the area is captured, players can either hold it or summon NPCs in to hold it while the players capture the next area. Once all areas in all regions are captured raid bosses spawn in each region. Each region can have mini-bosses that patrol it. Players can walk between capture areas and shuttle between regions.
Bring back slot machine decorations so we have a way to turn credits into reputation gains or cartel pack items.
A while ago you released a slot machine decoration that had a generous payout. A lot of people got bent out of shape so you nerfed the payout and didn't release anymore slot machines. I, for one, thought they were a great way to chill, chat, and work on my Contraband Resale reputation. You could also have events that revolve around these machines, similar to the now-running Nightlife event. Another option would be to tie them to the old/embargoed cartel packs. The prizes they award could items that dropped in the old packs - including faction certificates.
Add the ability to fast-travel to another player as a quality of life improvement and credit sink.
Sell compaion affection items on the cartel market to relieve the companion influence grind