Personally, I feel like blizzard may make the next expansion the last, but make it a very long expansion.
The following is a wall of text...
Say they end MoP on a note that Garrosh has been infected by the sha of pride to the point where he believes that the legion is no match for him, and attempts to open a portal to Argus to bring them to Azeroth. However, to do this he needs to know HOW. He searches for Azshara and they become allied because of a similar goal. Next, we kill Garrosh, and stop his attempt to bring in the legion. Once we stop him, we know that Azshara is next.
This is where the next expansion could kick in. The horde and alliance are currently working together to find Azshara, and take her down. The horde and alliance are not "allies" though. It's similar to the alliance they had in Wrath to take down Arthas.
We find Azshara, take her down, BUT we are too late. She has opened a portal, and demons are gushing through. Say this is where the regular raid ends. NOW for the hard mode... going THROUGH the portal. In hard mode, you could have to push the demons through the portal, and then walk through it yourself. Once inside, you find yourself inside the mountains, just outside of Mac'Aree (capital of Argus?) You would need to establish a foothold, and set up a perimeter and hold for reinforcements. Once you are hit by a bunch of waves, you would kill some bad ass demon. Once this raid is finished on a server, it could unlock the portal to Argus, and could be like the Isle of Thunder, where each server unlocks areas and progresses through a storyline over weeks. This could involve building a real stronghold, gathering supplies, destroying enemy siege weapons, and maybe even finding secret ways into Mac'Aree. Eventually, You push the demons back far enough that they have retreated into Mac'Aree. However, an assault on the city directly would be a slaughter. So that leaves the question to how you get inside? The answer is quite simple! You sneak in with a small group of heroes (players) and take the gate! This could be the 2nd raid. It would involve going through the lower parts of Mac'Aree and working your way up to the gate. Along the way, players could fight mobs and bosses. Since this is a stealth mission, the players are catching the demons off guard, and have to deal with only a small number (explanation for not needing an entire army.) Once the gates are opened, you get to see something we have been waiting for for a long time... Kil'Jaeden at full strength. The opening of the gate has drawn him out of his keep. However, being who he is, he never does his own fighting. He sends his minions to fight for him. BUT these are no minions. Kil'Jaeden knows we are indeed a threat, so he calls upon the dreadlords to come and slaughter us. Once the dreadlords are commanded to fight for him, he will retreat into his keep, to attend to more important matters. We kill the dreadlords, and it is considered a great victory for the mortals of azeroth. The outer areas of Mac'Aree are taken by our armies, and we begin to establish ourselves here.
NOW here is where it gets interesting. For the first time in WoW history, the next raid would not be based on an attack. We would be defending. This is when we learn that Kil'Jaeden is starting to get desperate. He has never had such a great threat to deal with, and is beginning to panic. Because of this, he has literally called upon the entire legion to attack us. This could start with the raid group inside Argus, waiting to be attacked. Now, we all know that the legion is so huge that you couldn't count its numbers. They hit us fast, and HARD. Now, before I go any further, let me attempt to paint this picture for you...
This is literally a WAR. This isn't like any raid we've had before, where we are just fighting with our raid group. We have an army with us. A very BIG army with us. However, the attack of the legion will literally decimate this large army, leaving few survivors. Those who do live (including the raid group) must retreat to the stronghold in the mountains where the rest of the armies are. The retreat could consist of a few parts. The first would be getting outside the gates of Argus, where you would be assaulted by somewhat weaker mobs (in a lore sense) that are massive in numbers. This could last to the point of where they nearly never come to a stop, and will involve the raid group being mobile 100% of the time. Once you reach the gate, you are met by Mephistroth, the final dreadlord left. He would be a fairly challenging fight, but not because of his strength, but because of the continuing ads pushing behind you. The raid would need to kill him, while killing the ads as well. Once this is done, the players walk outside, and the gates of Mac'Aree close behind them. This is where the raid would get a break to mana up and such.
NOW comes the 2nd part of the raid, getting to the stronghold in the mountains. This would involve aerial attacks from the legion along the way, but not at the speed/numbers of the first attack, but would involve multiple bosses. Once the players reach the stronghold, they are given a mission to help escort the rest of the army that escaped inside as well. This would involve moving injured soldiers, protecting healers, and making sure no demons get inside. Once this is done, the players get inside and the gates close. The final phase of the fight will involve the players being on top off the walls of the stronghold, still defending from the legions attacking, and would lead up to a very large pit lord coming to break down the gate. Players would need to kill this, but would be too late and the gate would be destroyed before his death. After the pit lord is dead, the legions attacks begin to diminish to the point where they can be held, but not pushed back. THIS is where stuff gets really deep into crazy theories. At the end of the raid. The players would be welcomed back by Wrathion, who has brought a friend along with him... Bolvar Fordragon, The Lich King.
It is explained that Wrathion has known about the imminent threat of the legion for some time, but did not believe the mortals of Azeroth could defeat it alone. It is revealed that he went to see Bolvar some years ago, and they began preparing the scourge to aid in the battle. This obviously would not sit well at all with both factions, especially since they were told that the Lich King was dead. However, because of their years of preparation and planning, Wrathion and Bolvar have experimented with the scourge, and have vastly increased its numbers, to the point of full recovery from the last war, AND topping those numbers.
This next phase of progression would be very interesting, simply because an area would actually be taken away from the players.
Now comes the moment of truth. The final raid of the expansion/all of WoW. The final attack on Argus. All of the forces of azeroth have gathered here with a single purpose... to destroy the legion forever.
The scourge forces take point, and actually make great progress, to the point of almost retaking the gate. Players will aid their undead allies in re-taking the gate and fighting off stronger demons, while the undead take care of the weak ones. The gate will be retaken, and the push will continue far past it. The players will push through the city, still aided by scourge, and will get to the stronghold inside of Mac'Aree. The initial attack will only need to be completed by a player one time, and the player will then unlock the actual raid.
Inside the stronghold lies Kil'Jaeden, who has not been seen since the the first attack. Players will enter inside to find a raid similar in layout to ICC, with wings, available in any order you wish. Once all wings are completed, you will get to the top, which is where you find Kil'Jaeden. The players find him opening a portal. It is then revealed that Kil'Jaeden has in fact gone mad because of the attacks, and has resorted to the last thing he would want to do... bring Sargeras into the fight (Sargeras has been MIA with the legion for some time.) Players will need to fight ads while Kil'Jaeden is opening the portal. To stop the portal from opening enough to let Sargeras through, they will need to go in themselves. Imagine this part similar to fighting Yogg'Saron, where you go inside the brain, kill some ads, and then kill something with a lot of health (being the portal itself.) Once the portal is closed, the players are joined by Bolvar and Wrathion, who have secured the rest of the area. This is when Kil'Jaeden begins to fight you himself. The fight will involve using Wrathion and Bolvar to aid you with certain buffs the grant you, to keep immune to deadly spells that Kil'Jaeden casts. Once the raid gets him low enough, the raid will see something happen that will make them shit their pants... he will heal to full, BUT he will also be stronger... much stronger. The raid will have to do the fight again, but he will hit harder, attack faster, and cast even worse spells. This time, once he gets to a certain health, he will be aided by the remaining demons on the planet. This will result in the players being VERY distracted with the demons, and will be unable to attack Kil'Jaeden. THIS is when the Bolvar releases his secret weapon... a plague made to kill demons. The players learn that this plague has been in development since the first Lich King, with the goal of stopping the legion. However, it was never perfected and they never found a way to make it work... until Bolvar came along. The plague kills the demons, and greatly weakens Kil'Jaeden. The players then kill him, with Bolvar getting the final blow. Kil'Jaeden is killed by his own creation.
This final boss leave the option for Blizzard to make another warcraft game if it was ever wanted, but the threat against azeroth is gone... for now...
TLDR; Garrosh allies with Azshera. Kill Garrosh. Kill Azshara, who has opened a portal to Argus. Go to Argus, kill shit and build a base. Sneak in to Mac'Aree, open the gates, get armies inside. See Kil'Jaeden, kill dreadlords here. Legion attacks, pushes players back to the base. Wrathion pop out of nowhere with Bolvar and kills shit. Scourge helps re-take Mac'Aree and the players go into the stronghold inside. It looks like ICC inside. Players kill more shit, go upstairs and find Kil'Jaeden opening a portal for Sargeras. Players stop it, fight Kil'Jaeden, almost kill him, he heals and goes Super Saiyan on the raid and sends demons after us. Demons are high in numbers and stop us from attacking him. Bolvar releases a plague that kills the demons and greatly weakens Kil'Jaedan. Players kill Kil'Jaeden. GG