Ok..has happened to every expansion so far where a raid is impossible to solo so blizz had to nerf it so people CAN solo it....
Vanilla:
BWL Razorgore
AQ40: Twin Emps/C'thun
BC
Black Temple: Reliquary
SWP: Kalegos
Wrath
Ulduar: Thorim
Cata
Dragon Soul: Spine of Deathwing
Mists
Siege of Org: Galakaras
Warlords:
Hellfire Citadel: Hellfire Assault, Gorefiend
So what's next? Obvious one for Legion is Eonar and for BFA N'Zoth but...not sure what else might end up being problematic for soloing it when its trivial.