Yes i might be saying what people already know, i'm not that deep into lore, i've just read a bit here and there. But i though i'd give my two cents anyway, this is how i think the storylines are going to be played out.
1-> We stop the legion invasion for now, maybe even fight the avatar of sargeras, maybe not. But this is confirmed to end the threat of legion we need to take the attack to them.
2-> We find more about elune and the titan that is Azeroth or currently on Azeroth, get some light shed there and maybe even activate the titan in the coming expansions, but for that we need to get rid of the corruptions within.
3-> After the first stop, massive legion outbreak somehow again and this time Nzoth weakened and along with the nagas show up to battle, we need any allies that we can to withhold the attack of the legion in azeroth. (This is mostly taken from another speculation thread, will add source later)
4-> We get a final tier raid with the legion attack mixed along with naga and nzoth, who have their reasons to fight against the legion as well. We get to fight the avatar of Sargeras with endless armies of the legion invading and that's the time we get to see our death vision we had from HFC and in that duration we are almost defeated, if it wasn't for Nzoth who starts devouring all the legion army and taking their power untill the avatar is greatly weakened and we finish it off.
5-> Where we think the legion threat has been stopped for now and we could actually go and regain our power, that's the time when Nzoth who has now been empowered by devouring the legion armies and a bit of the avatar of sargeras has taken hold. I'm not sure how they can play this out, but i'd love to see an instanced Kalimdor or Eastern Kingdoms ( Would love if they did both ) which is all zone-scaled and has been invaded by the terrors of the final old god and queen azshara. They could do this bit by bit, like the plague begins. Lots of the inhabitants of the areas starts getting infected by the old gods and slowly turn into their followers and they could also go down the zandalari storyline along with it. This would be a kick ass expansion.
6-> This whole expansion, we fight off the old gods threat once and for all and discover about the titan that azeroth is or the titan that azeorth beholds and how do we get it activate, but to do that we need to fend off Nzoth and the Queens armies. We end up killing nzoth with the help of several heroes throughout, meanwhile queen azshara escapes to argus and teams up with sargeras again.
7-> After the corruption of azeroth is finally over, we get a confrontation from the real threats, the badass, the shadow, void-lords and we understand how the old gods shit happened and how powerful void-lords are above the titans.
8-> We understand the gravity of the situation we are at, one side Sargeras and the burning legion who won't stop invading and on the other side the Void-Lords who we know very little about, but understand that the threat is imminent.
9-> We now need the most powerful entities in the warcraft universe to fight these threats and we need to take the fight to them. And that is how we awaken the titan that is on Azeroth or is azeroth. It's hindered by some thing yada yada, but mostly it gets activated and now it's time to take the fight to argus.
10.-> At this point i feel, blizzard can go ahead in the direction of Titan mmo or WoW 2 even, but if they don't i see an awesome 2-3 expansions on invading Argus, the stronghold of the legion. We can even get to see ethereals playing a huge role amongst other gazzillion races they can make or use.
I know my speculation might have a lot of plot holes and probably wouldn't make sense, i hope it did make sense lol. This is not structured properly, just vague thoughts put on as points, i hope you bare with it.
But yeah i'm no lore master neither am i too much engulfed into warcraft storyline, i've just started out recently reading/watching videos and getting to know stuff about the warcraft universe.
This is just my amateur speculation on how things are going to play out and if it does, i'm quite sure we are in for an epic adventure.