Okay, so 8.3 will most likely hit the PTR in a week or two, so let's make your bets. Where will the final raid take place? I tried to gather most popular opinions. This poll also includes new zones, if there'll be any.
1. Silithus Raid (no new zones) - conclusion of the central problems of the expansion - we have a Titan dying. So, this will be an actual Battle for Azeroth. But this time, it's a different meaning of "for". So, we'll have to deal with Sargeras' sword somehow, heal the wound, stop Azetoth from dying and not let all the bad guys take her. What bosses should we expect in that case?
Servants of N'zoth, qiraji probably, some azerite elemental. Also possible to see some demonic boss because of the sword.
Possibly Nathanos, if the mission Sylvanas has sent him to deals with killing Azeroth and raising her into undeath.
As for actual endboss, there are: most likely N'zoth's tentacles and servants, protecting Azeroth from them.
But Eonar-style boss is an extremely bad idea for and enboss. It can be Azeroth herself, already darkened. So, we defeat her and cleanse with some cool new medicine from Magni\Wrathion. Also can be Azshara, as a vessel for N'zoth's power, but that is extremely bad too, you know why.
Another possibility is Sargeras, if he manages to make a portal via sword from his prison. That would be epic, but chance is very low.
My bet on this variant is that when we save Azeroth, she smashes N'zoth tentacles and he flees to Ny'alotha, which appears in the next expansion.
2. Ny'alotha Raid - most likely this will happen. As a contra, it leaves question of healing the wound unattended, and wastes N'zoth, who was being built up since Cataclysm. Yes, Ny'alotha will be a huge lot of content, and we had lot of hints that next fight is against Azshara's master. But... it Legion we were repeatedly told that it's time to fight Sargeras, but we didn't get to fight him directly. And anyway, Azshara has to die before her master, so in any case she will be fought again if we kill N'zoth in BfA. And another problem. N'zoth is, well, big. Really big, so no idea how do we fight him. On the other hand, we killed a titan in the last expansion.
3. Stormwind Raid - I don't know why is this even here, but many people still believe it can take place. With Sylvanas, who now has next to zero followers. Still, someone may still believe in that ridiculous Shadowlands leak, though it's already proven wrong by 8.2.5 plot.
4. Dragon Isles Raid - unusual and rather interesting version. May turn out to be a good and legit compromise. Too early to introduce Ny'alotha, but still a new zone. Wrathion is said to be there at the moment. Dragonflights are receiving more developement recently. Weaknesses of this variant - no idea how to put together N'zoth and Dragon Isles in a form of raid that would actually have him killed or imprisoned.
My guess is that Dragon Isles, Ny'alotha, Shadowlands will all come as zones in the next expansion, now our main purpose is to save Azeroth from dying, have her wound cleansed and healed and not let N'zoth, Sylvanas, Legion and whoever wants to harm her, win. That would sum up the idea of expansion which started with a war that split and devastated the war, releasing and Old God eventually, and finish with all races united in peace to protect the world from the true enemies.
Still, anything is possible, especially a waste of a long-awaited villian in a single patch after his release.
Make your bet.