Top 5 in order for me:
1) Class Story
At least another chapter in similar vein to the existing chapters 1,2 and 3.
Not necessarily new companions but expanding the usability of existing ones. More interactions at end game, allow player to choose their role, open up their starting gear as orange modifiable sets, once unlocked allow them to be shared across legacy.
3) Other: Better ingame communication tools
Legacy address book, ability to friend other faction. I meet new friends and them lose them when they go to play an alt
4) Other: Better Guild support
In game resources such as calendars. Never understood why the pre-launch guild rosters and forums disappeared. The ability to add a legacy rather than individual and the ability to filter by legacy in roster.
5) Other: More customisation options for existing classes
New Advanced Classes. The ability to use a wider range of weapons. The ability to swap Advanced Class.