I think it will be a pseudo-reverse of Wotlk. During Vanilla we were just adventurers exploring the world. During BC we were arms for the Horde and the Alliance fighting a common enemy. In Wotlk we became heroes and attempted to be recruited as an 'A-Team' force that would be turned against the living. In Legion I think they want to kill this 'A-Team' right out. In WoD we had little forces and supplies but annihilated an army more formidable than the Horde in the first and second war with a few thousand troops, mostly elite soldiers/heroes.
The Legion will have recognized that all their attempted into Azeroth such as Aegwynn beating Sargeras, Khadgar and the gang beating Medivh and eventually pushing the Horde back into the portal. Malfurion, Tyrande, Thrall and Jaina holding back the Legion during the Battle of Hyjal. Cenarius, Malfurion, Illidan, Tyrande and other animal gods dominantly holding back the legion during the WotA. You get the idea.
I do expect a large culling, but I think the Legion's initial plan will actually be to kill off major players in Azeroth. Also that the Horde and the Alliance are held together by these leaders (dominantly the Horde) and by eliminating them it gives the potential for the two factions to fight against each other and possible civil war. With the world weak and leader/heroless the classes band together for a common goal since the factions cannot work together because they are both in shambles.
After the culling we become the new class leader ie replacing Tirion as the Paladin leader, Thrall or Rehgar as the Shaman leader, Khadgar as the Mage leader, etc. It is speculated and I agree that many of these heroes will die or will go back into the hiding such as what Tirion did. This is also supported that we get many of the Lore heavy artifacts.
In the end we will become superhero status such as becoming as strong as Khadgar by the end of the Legion invasion and pushing them back. However it has broken up both the Alliance and Horde, plotting against each other like Sylvanas the next LK and expanding the war against the Worgen, Lorethemar and BE splitting or more talks with the Alliance. Possibly with Demon Hunters and Mages now in NE society there could be a civil war or xenophobia. A lot of possibilities.