Any speculation as to what the middle tier raid will be?
Historically speaking, the mid patch is often a raid noone sees coming or is unprecedented. In WotLK we had Secrets of Ulduar, Cata - Rage of the Firelands, MoP - The Throne of Thunder (all 3 EXCELLENT instances, even with Firelands only having 8 bosses).
Obviously, WoD had it's mid patch cut. (My bets were on Shattrath). But let's NOT talk about WoD.
Raids in Legion:
Nightwell and the Emerald Nightmare for the first 'tier'
*unknown* second tier
Tomb of Sargeras, obviously, as the final tier
My personal speculation... It's an ideal time, given the Azshara lore, heavy night elf history and naga prescence that we'll see the Abyssal Maw raid at last. It was promised in Cata as a twin raid with the Firelands in Cata, yet as we know it was put on reserve when it would better suit the story.
Other options... N'Zoth instance. Given the continuing presence of the old gods even within Ulduar to the present and that our characters are apparently being manipulated by unseen hands, it's possible part of that resolution and our final 'breaking of the cycle' could take place.
Troll and Naga raid at Zuldazar. Given our quest for the pillars of creation, the fifth piece may not be on the Broken Isles, but with the Trolls on Zuldazar, which is also an island which survived the sundering and implosion of the Well of Eternity 100,00 years ago. Plus, this is the Troll capital (where they meet every 6 years... and we're looking at year 12 of WoW... conspiracy theories go!) Plus who doesn't love trolls?!