The one thing I want above all else is more hotbars. 48 slots simply isn't enough for all of a level 50's abilities, relics, stims, adrenals, PvE and PvP health packs, speeder, travel abilities, "throw the huttball", buffs (which could be 4 soon).
After that I'd like more operations. Larger, longer, more fun, ideally for 32 people but I know that's a pipe-dream, with a new tier of gear. Something involved like a series of trials that you have to get flagged from in order to progress, akin to the Muramite Proving Grounds and unlocking Citadel of Anguish in EQ1.
And finally cross-server PvP queues to keep the wait times down.
Don't give a stuff about ranked PvP, Flashpoints.
Oh and the whole 'bring a friend' mechanic... get rid of it. Use aggro ranges and let us have some proper pulling abilities that make or break raids.