There have not been many interesting blue posts lately. Everyone was waiting for the 4.3.2 patch, and further in the future even the MoP Beta. That's why I think it is in order to think about who can we expect as the Mists of Pandaria end-bosses.

There have not been official announcements on who are we going to face in MoP raids. All we know is that Blizzard will focus again on the old conflicts, that is, the Horde vs. Alliance war.

In the 'early' expansions, up to and including the Cataclysm one, we always knew who was going to be the one that we were supposed to fight in the end.
When The Burning Crusade was released we already knew we were going to face bosses like Kael'thas Sunstrider, Illidan Stormrage and Archimonde. Kil'jaeden was a bit of surprise, but we knew all of the other ones.
In The Wrath of the Lich King it was even clearer to us. We had to defeat Arthas aka. the Lich King. There were no surprises here and everyone at least saw the last boss, even if they never actually cleared Icecrown Citadel and defeated the Lich King.
Cataclysm brought us Deathwing. It was a known fact right when this expansion was announced.

When the big announcement about Mists of Pandaria came at BlizzCon 2011, we learned something about a few upcoming class changes: we saw the new talent trees, we previewed Pet Battles, PvE Scenarios and Challenge Mode Dungeons, even a few things about New BGs and Arenas were revealed, but when it comes to dungeons and raids, where most of the end bosses are, all we got were just some general notes. No names, no lore... nothing.

So, beside the revamp of Scholomance and two wings of Scarlet Monastery, we also heard about a new dungeon called Shado-pan Monastery in which we are going to meet the Shado-pan Clan. There are going to be nine dungeons in total, but that's all we know about them until now.
As for raids, we are going to face three new raids right at the launch of MoP, and world bosses are going to return too. Mogu'Shan Palace is a new raid instance and it was told that it holds the old Mogu Empire secrets.

Now, there are two things that we will focus upon: the Shado-pan Clan and the Mogu Empire.
All we know about the Shado-Pan Clan is that they are a pandarean race, but they are very different from the other ones. They are darker and more sinister creatures, with a ninja like culture, but dedicated to protecting Pandaria. They fight an endless war against the Sha.
What is Sha, you ask? Well, Sha is the living manifestations of negative energy, and since the most negative energy was brought to Pandaria by the Alliance and Horde, with their never-ending war, I honestly don't believe the Shado-Pan Clan is going to be very happy to see any of us.

Now for the Mogu Empire, they inhabited Pandaria in a distant past. The Mogu were powerful magical creatures that ruled Pandaria and held its inhabitants as slaves. They ruled with a no-weapon policy, but the pandareans managed to dispose of them and establish their own empire. Now, hiding inside their Mogu'Shan Palace, they are planing to take over Pandaria once more.
Mogu means, roughly, "negative ancient" in Chinese. That means that the Mogu are not likely to be welcoming creatures either.

Both the Shado-pan Monastery and the Mogu'Shan Palace are located on Kun-Lai Summit at the center of Pandaria. At the top we can find the Temple of the Tiger along with the White Tiger Clan. That clan is going to be a new faction, pretty much, and it has not beed revealed yet if we are going to face them in battle.
The interesting thing is that the Kun-Lai Summit is threatened by Zandalari Trolls, invading from the north. That means we will have to deal with trolls once more, even if Zul'Aman and Zul'Gurub are now history.

Based on these data, we already can see four potential enemies: the Sha, the Mogu, the invading Zandalari tolls, and (maybe) the Shado-Pan Clan.
Unfortunately there is still very few information around to make out a final boss figure out of this. But, there was an interesting picture on the web a while ago:

That is a screenshot from Descent into Madness, from Ulduar, and as others have said, that stained-glass window resembles a Sha creature. Because the location of that window is near Yogg-Saron, and old god, it's not very unlikely for us to face another Old God or something similar to it.

Now the following question is pretty obvious: What/who do you think are we going to face in the next expansion?