I'm sure this has come up before and I don't ever cruise the forums so I apologize. And to be clear...IM NOT ASKING TO ALLOW ADDONS FOR SWTOR...least not the way WoW did it. That being said, there were a few addons from WoW that were immensely useful and wondered how I ever played the game without them. WoW actually took notice of some of the addons and implemented them into the game itself.
1. An equipment manager/auto-switch - this should've been implemented at the same time Field Respec was introduced into the Legacy perks. Probably should've just did a copy/paste of WoW's dual-spec & gear switch system. And probably should've had this at game launch.
2. Going to get flamed for this but Recount was useful for a lot of people and not just for stroking ego's. When I play, I want to play well and Recount helped me decide if I needed more of this stat or that stat. All anti-addon folks love to scream how bad Recount was for the game but in my opinion, it made me a better player. It also shed light on the lazy players trying to be carried and score HUGE upgrades with little effort.
3. A better, more customizable UI - Please take a look at the Domino's or Bartender addons if you do consider revamping TOR UI. At first, TOR UI is really cool looking but when you start getting more abilities and doing more intense game play, less is more. Player frames and quickbars, especially the Main Quickbar, needs to be leaner. My personal beef with the current UI is the Player Frames. Would be super if I could change the dimensions of the health/energy bars without making the player portraits grow exponentially along with it. The buffs/debuffs also need to be customizable on player/enemy frames. It's hard to tell if you need to refresh dots/hots on a target when many of the icons for them look exactly like yours.
4. MiksScrollingBattleText - I cannot stress enough how awesome this is and to be honest, TOR's floating text is horrible. This addon created separate frames for floating text that were totally customizable.
5. I forget the name of the addon but it was something even WoW had implemented into its game and made me scratch my head as to why I never had this from the start. When one of your character abilities had a proc, the addon would play a specific sound and/or flash a small graphic above your character, letting you know the procced ability was ready.