My top 5 list:
1. Class stories
Many of them ended on somewhat of cliffhangers and I would like to see what happens next. Also I''ve read somewhere that the devs find it hard to get all 8 classes a reason to be on the same planet. Wich although haven't stopped them pre-50, I don't get why every new chapter has to be on a new planet. They could jump from planet to planet on already existing planets f.ex.
Also liked the idea posted early in this thread about releasing it periodically, like f.ex telltale does with The Walki8ng dead. For me it doesn't have to be level cap raise either with class story. Can keep the cap raise with the overall story like Makeb was for my sake.
2. Minigames
I would love both podracing, swooprace, pazaak, sabaak an much more. Make Nar Shaddaa the epicentre for gambling and games.
3. Planets
Would love to see some excisting (in star wars lore) planets andtotally new ones. Expansions with several new planets.
4. Companions
Both new ones and develop those we have further
5. Space thingy
I like both pvp and pve so more of both never hurts, therefore i would love for them to include a PVE option for it. Also with this would be cool if they made ops, fps and such where you have to attack in space, and then continue on ground/inside ship/spacestation after f.ex.
I voted for almost all of them but these are my topses
Also, for the discussion going back and forth here, it has been my understanding from all i've seen leaving that i know of, that it was never the class stories that was the reason for them leaving. The reason for them was lack of endgame, or too easy endgame, plus lack of QOL things. In fact, those i've talked with have all said SWTOR has the best levelling experience BECAUSE of the class stories that they had in any mmo.
That is not all players who left probably though.