So far we've seen Nefarian return as the "end boss" of Cata T1. (Sin is a "special" heroic only boss so it doesn't count.) Now we have Ragnaros as the end boss of Cata T2. This whole expansion seems to be themed around trying to bring back the feeling people had in classic/vanilla wow. They revamped old world, and brought back a couple old end bosses. Lots of focus on old things and bringing old things back (za and zg too) I fully don't expect to see a completely new and fresh tier end boss this expansion. EDIT: Besides Deathwing of course, forgot about that when I was making the OP >.<
Which tier end boss do you think they'll bring back next? I don't really think they'll bring back Kel'Thuzad YET AGAIN because the whole scourge storyline kind of climaxed with Bolvar. The usual "plaguelands, northrend, and killing the lich king were merely setbacks..." doesn't really fit here, but Ner'zhul could still corrupt Bolvar and start another uprising of the scourge and return Kel'thuzad to the world of the living again I suppose.
C'thun is also a possibility I guess.
The bosses from upper and lower blackrock could make a return I suppose, though it doesn't make much sense story line wise. Also we could see Ossirian the Unscarred, with Uldum being "unearthed" and all. Pretty much leaves nothing else but burning crusade bosses, which are probably on reserve to recycle during the Sargaras expansion that will likely be wow 5.0 (to end the storyline and try to get everyone into Titan/Diablo3)
So what are your thoughts and opinions on what we'll see in 4.3 and beyond?