So we know what 5.2 has to offer, a legendary meta gem and 1 more socket to our sha touched weapon. This raises the question that I am sure has some significance, will 5.3 OR the Siege of Orgrimmar offer the actual proc and the transformation from purp to orange? Most if not all legendaries have had their entire "building phase" put into a single patch, and usually required tons of gathering or lucky drops or a single instance/tier to assemble.
It feels already that the sockets barely compete with the 522 weapons and higher that will be coming out, also to mention thunderforged etc etc, now we are left wondering how the last chapter of this legendary quest will pan out.
From what I can tell, and this is entirely speculation but with general "blizz would probably do that," added for flavor:
We'll start off the instance by having to gather 20-40 of some sha/elemental/corrupted essences from Garrosh's forces for either another socket or a proc added to the weapon. Presumably the earlier since we get a meta gem in tier 15, but of course its variable.
Followed by the final part which may be the actual "upgrade to legendary" involving killing BLANK, getting BLANK's HEART or something as obvious and recieving the item by turning in our sha-touched for the actual clean orange. This could also prove to be a twist and have Wrathion turn against us and recieve some just flavor reward, but that's speculation that I'd save for when were on the ptr for last patch of this expac later this year.