The way the story goes right now is heavily pointing towards an Old God Tier for 8.3...imagine the joy of the raiding community if they get Crucible of Storms x4 for the last year of the expansion.
Of course, it's also possible that they directly jump to Sylvanas' Assault on Icecrown and the last tier will be undead-themed....but that would be a complete waste to throw at this garbarge expansion.
So, imho, 8.3 will be a horribly boring, unfun Old God raid..which is a GIANT mistake after rushing in a raid that COULD have been awesome like Crucible of Storms, just to make sure that everybody will already be burned out by the theme before the raid has even opened.
Maybe Blizzard will manage to actually surprise me in a positive way with the 8.3 setting...who knows? It's possible. But i don't realistically expect it to happen. I bet my money on a horribly-received Old-God tier that heavily reuses Crucible assets, which already is one of the least popular raids ever at this point...and that will be the grand finale of BFA for almost a year until we follow Sylvanas to Northrend in 9.0. If this actually holds true, i have no doubt in my mind that BFA will go down as the worst expansion in the history of this game...easily beating WoD to the title and letting Cata look like a marvelous gem of Video Game design.