I am fairly certain the next WoW expansion will deal with the legion as the main antagonist if not a major role, possibly going to a planet like Argus. Speculation aside, this is how I believe would be the best way to make new content and keep all levels of players in the old content.
Much like WoD, I would imagine the legion comes across some artifacts and then finds a way to invade us again and is able to make some portals to allow legion agents in. The idea is each neutral territory (Ashenvale, Wetlands, Westfall, etc) would have a random legion agent invade the territory. This would act a lot like world bosses except they are not there waiting for us to attack them, they actually invade with a small force and will continue to spawn more troops until the main boss is killed. Also if left unchecked the boss will take over multiple towns.
Despite being able to take over towns, a player would be able to solo the boss through special mechanics. Mechanics like getting on a town’s turret gun or going off to NPC patrol groups and telling them the town is being invaded so they can support you in killing the boss. However I feel using mechanics should take longer and harder, but doable by yourself. When the bosses are fought as a group of players they are killed easier and quicker than using other mechanics.
To compare this, think of the quest “The Last Line of Defense” quest type of Legion invasion while the bosses are comparable in strength to Stitches, relative to the zone.
The reward for killing these bosses would be similar to finishing a dungeon (a generic blue item) and a repeatable quest item which you turn in for exp. At higher levels there would be max level demons ( 4 total; one in lower Kalimdor, one in upper Kalimdor, one in upper East Kingdom, one in lower East Kingdom) who allow the smaller agents that invade the leveling zones to enter Azeroth by opening portals for them. These max level demons would not be invading as they are there to sustain power for other agents to invade. In this sense we would need to invade on them. They would be comparable in strength to the Timeless Isle bosses and of course would give rewards or possible weekly quest. These bosses would eventually be summoned back into the world from the main source of power which allows the legion to be in the world and it would be the dominant cause of the expansion to stop the flow of this power, much like we did with the Sunwell.
Of course when a larger boss is killed no invasions can made in the leveling zones. To compensate a regional buff should be granted for all players which gives +exp(10%?) and damage until the boss spawns again. Major boss spawns could be every 2 hours while invading bosses could be every 30min.