1. Class Story: Class specific orientated stories that add the extra mix-up that actually makes me want to create new characters regularly, many mmos I'll stick with one class and character since the game plays the same 80% of the time, SWTOR has kept me coming back due to the class storylines. Obviously that only really lasts until SOR expansion though which is disappointing, I try to take a character through the rest while leveling a new toon to keep my sanity. I would love to see additionally class story even if it's one quest like Rishi where I'll want to take an alt all the way through in one go.
2. Companion Stories: One of my favourite parts of this game is not only companions being around that make soloing so much more fun than other MMOs, it's the sheer detail that went into the companion stories, I will level up affection on every new toon and will listen to the convos and do the quests on all new characters. I always hoped they would add companion quests to the other class specific companions in addition to your first companion but it never came to be. One of the worst things they did was 'remove' companions in KOTFE and thus the built up stories, I haven't taken all classes through KOTFE yet but I was disappointed at the lack of continuation of companion specific stories.
3. Solo Space Missions: These were fantastic to play when I got into the game when it became available in Australia, at the time server stability sucked which made any form of grouping beyond flashpoints a high pingfest and it was great playing these missions. I would probably spend as much time grinding these as playing the rest of the game and it sucked getting GSF and not an expansion to this mode. I could catch up on shows, comics and manga while playing SWTOR and not having to keep stopping for cutscenes.
4. Flashpoints: I may be a solo centric player but I do enjoy grouping and teamplay at times, what ruined them for me for a long time was Spacebar idiots in storymode flashpoints and rushing/skipping most of the fights just to rush to the end. One of the best things they did was make many of these flashpoints solo so I could actually enjoy myself and not have my time ruined by impatient players, if they were to expand this further to other MMO content I would be even happier. Before the solo mode being added I would solo or duo these alot and that was way more fun than other players despite more deaths.