Well the first step in this is to release the information required to develop in their addon language. I personally had no idea that they were going to allow player created addon's this time around. I am assuming that the Beta has no support for these in P2. Do we know if they will be supported in P3?
Its interesting to note that a game like Wildstar has addon support as well, and it took less than 24 hours for a beta tester to create the first addon for that game without any documentation of the game's UI scripting language.
I for one, hope that its sooner rather than later, if for no other reason to allow me to not have the gargantuan spaces between my action bars/buttons. And maybe numerical CD timers on buttons like were there in 1.0. From all the screenshots I have seen they are quite annoyingly centered and quite frankly look like they can be in the way.