I got burned significantly when I tried to imagine what the "Middle Tier" of Warlords of Draenor would be. As we all know, there was no "middle tier." Instead, we just went directly from Blackrock Foundry to Hellfire Citadel. No Infinite Dragons, no Farahlon.
So that's my big grain of salt in all of this. I do think (or at least strongly hope) that Blizzard has learned from their mistakes in Warlords - that not only do players want to feel like they've got a whole expansion's worth of content, but also that an expansion as significant to the lore as Legion deserves to have a large amount of content. I'd be very excited if we could see the kind of endgame tiers of Wrath and BC.
But if they have learned their lesson, and we're getting at least three raid tiers, that opens up certain possibilities.
The best way to go about this is to figure out what all the significant threats are in the expansion.
Number One - in the expansion and the game as a whole - is the Burning Legion. We don't know just how significant a victory we're going to win at the end of the expansion (it'll have to be at least something of a victory so that the game still exists afterward) and who the final boss will be. Given that we just beat Archimonde (again) in Draenor, it seems like they've got to give us either Sargeras himself or the Avatar of Sargeras, or something of an equivalent power level (if such a thing exists.) If Kil'jaeden is a boss, I doubt he'll be the final boss. Honestly, I think this is going to be the expansion where we finally take on the Dark Titan himself, even if we might not totally kill him.
The Nightmare is also going to be a significant part of the expansion, but I doubt it will continue to be a factor after the initial raid.
Azshara's Naga certainly have a presence in Azsuna, but the question remains whether she will be a presence herself. I could imagine Azshara being a mid-tier boss, but I honestly think that she's a big enough name that they could build an expansion around her.
The Nightborne will likely be dealt with after the Suramar dungeons and the Suramar Palace raid, even if Gul'dan is the final boss, we'll likely see the end of them as a significant threat.
The Drogbar I really don't imagine being a threat after we do the Neltharion's Lair dungeon.
Likewise, while I'm sure there will be some interesting lore surrounding them, I don't think that the evil Vrykul will last much beyond the two Stormheim dungeons. God-King Skovald will go down in Halls of Valor while Helya will probably fall in the Maw of Souls.
We do know that there's another zone called Thal'dorah or something southeast of the Broken Shore, and it will open up in a later patch. We still don't really know what is going to be there, but I suppose it's possible we might get some insight into Elune and maybe see if she's connected with the Titans.
It seems likely to me that the Tomb of Sargeras will be the final raid of the expansion. It's possible that the second raid will be in Thal'dorah. Alternatively, we could have a raid elsewhere in Azeroth. While I really hope that any new Karazhan instance leaves the old one as it is, it might be a good spot for a new raid. That said, there have been some comments that seem to imply they're thinking of making a new dungeon (or dungeons?) there, so that might be its primary role.
I'll have to see the basic stories of each of the zones to get an idea of whether there are any other significant threats indicated in the story of the expansion. But so far, this looks like the main spread of villains (there's also apparently some big rivalries between Genn and Sylvanas, but I don't know if that's going to be reflected in anything other than quests.)