i personally doubt we'll face Y'Sharrj in the flesh (or whatever old gods are made of), seeing as how it's been described as "dead dead"
y'sharrj will cause more indirect havoc imo, but seeing as how the sha story is supposedly wrapped up im guessing it will have something to do with the bit of y'sharrj that has just been dug up
old gods CAN die, however it takes some serious titan-themed punishment in order to properly kill them, however us mere mortals can only delay them due to the fact we lack any weapons powerful enough to kill them (excluding re-origination because that would kill us too)
Also, when you kill the Old Gods, theid deaths bring pain and chaos into the world (in case of Y'sharrj the Shas) that's why the Titans didn't kill the rest of the Old Gods, cause that will mean the destruction of Azeroth
Yogg-Saron was kind of a side-story to the whole LK thing in WotLK, even if it was by far the best story in the expansion..
agreed with being the best story in WOTLK, however, i thought that yogg-saron played more of a main-ish role in WOTLK, as he were included in plot lines in numerous zones (grizzly hills and zul'drak being the prime 2) the 1 quest in zul'drak where you gather some saronite ore to get it destroyed since it were infused with yogg-saron's black blood, hense when we killed off (or rather delayed him) yogg-saron we esenssialy cut off the lich king's supply of tempered black blood saronite, to which afterwards with the icecrown 5 mans, we finished off the rest of the supply the lich king had.