Let's look at different raids throughout WoW's history:
Vanilla:
MC - a fire elemental cave
OL - a dragon's lair
BWL - a dwarven citadel occupied by dragons
AQ20 - an ancient titan ruins occupied by bugs
AQ40 - underground tunnels occupied by bugs
Naxx40 - a floating undead citadel
TBC:
Kara - a magician's tower
Magtheridon's Lair - a dungeon with an imprisoned demon
Gruul's Lair - an ogrelord's cave
SSC - a naga cave
TK - a hijacked naaru floating citadel
MH - a battlefield vs demons
BT - a draenei temple
SWP - an elven fortress occupied by demons
WotLK:
Naxx - a floating undead citadel
OS - a traditional dragon's lair
EoE - a magic dragon's lair
Ulduar - a titan facility
TotC - a tournament-style raid
ICC - an undead citadel
Cata:
BoT - a cultist citadel
BWD - a dwarven citadel occupied by dragons
TotFW - an air elemental citadel
FL - fire elemental lands
DS - a battlefield vs Old God forces
MoP:
MSV - ancient mogu ruins
HoF - an insect citadel
ToES - mogu ruins occupied by the Sha
ToT - ancient mogu ruins
SoO - orcy orcs
WoD:
HM - the capital of the ogre empire
BRF - orcy orcs doing orcy things in an orcy stronghold
HFC - orcy orcs doing orcy things in an orcy stronghold with some demons thrown in
Legion:
EN - Old Gods doing things in a forest
ToV - an ancient floating titan stronghold + some undead vikings
NH - an elven palace with some demons thrown in
ToS - elven ruins with some demons thrown in
Antorus - more demons
IMO SoO - BRF - HFC arc was unacceptable because it was thematically the same raid over and over again with slight variations in color scheme and design. HFC - NH - ToS - Antorus is shaping up to be even worse with demon bosses in purple and green strongholds over and over again.