Horde, once again united as strongly as ever, kick the Alliance the hell out of Orgrimmar. Then a huge anvil drops on Aggra, crushing her into a bloody pancake. Thrall is enraged by this, tears off his bathrobe and totes the Doomhammer armor once more. Everybody bows before the renewed true Warchief and Vol'jin founds a club in Cleft of Shadow, where he comes out of the closet and is openly transsexual, conducting drag queen comedy shows for the customers' entertainment. Lor'themar, having found his nuts in the previous patches, takes it up to himself to create a proper blood elf district into Orgrimmar, which is built next to Valley of Honor, serving simultaneously as a proper harbor for the Horde.
Politics-wise, we're back to Cold War, but there are some quite riled up feelings about, so one can imagine the H-A war hasn't left definitively.
While the leading army of the Alliance and Horde are located in and around Orgrimmar, when they just r3eched victory against Garrosh' forces, Sargaras commands Queen Azshara to sent her naga army to the shores of Westfall and Tirisfal Glades to attack both Undercity and Stormwind now they are vulnerable. Naga's succeed in invading through the frontgates.
To hold off most naga's the towns of Brill and Gold Shire forms a strong resistance against these them and that's where you as player can accept the pre-expansion's event quests, until the last phase of the event. The last event takes place in Stormwind and Undercity. Because some Naga warlords are rampaging important places in both cities.
When the High Warlord of those Naga's is defeafed you get a clue that Sargaras is behind this attack and your faction decides to investigate the attack and search for clues at the Naga's capital of Nazjatar. This the first zone of the next expansion. Horde starts their quest at the Broken Isles, while Alliance starts on Kul Tiras with their questline. While most of the adventure is above the sealevel, the final quests of the zone takes place underwater.
After Nazjatar's defeat players are sent to Sargaras homeplanet, called Argus. The adventure continues there.
i would like it if it was a stupid mass effect 3 plot.
where all along garrosh was the hero that was protecting us from the BL but we fucked up and killed him
Here we go friend. So after uniting all flets.. i mean races thrall enters orgrimmar with entire army. While shit is going all around he looks at place where pre cata kaja's closest to AH shop once stood and says "im warchief thrall of frostwolves and that was my favourite shop in orgirmmar". So after this shit explodes and he enters garrosh fortress. So just before he enters throne room, vol'jin appears and is like "we need to destroy garrosh !!!" but then wrathion enters and is like "we can controll hellscream think of possibilities !!!!". But since wrathions head is much smaller than normal thrall is like "lol you are indoctrinated by garrosh" and then uses his maxed speechcraft to force wrathion into commiting suicide. Meanwhile vol'jin dies for no reason just to make things more dramatic. So just before confrontation with garrosh anduin appears but now he's made out stars or other titanic shit. And he's like "there is 3rd way thrall - symbiosis with garrosh" and so thrall merges with garrosh making all races of azeroth half garrosh (with tiny heads). Here is your ME3 style ending.