I consider it to be part of WoW 3. Legion was the start of the current reboot and BFA continues that. These last 2 expansions definitely feel like a new era for WoW. Legion fixed a lot of the internal shit and BFA seems to be fixing a lot of external shit. These last 2 expansions make WoW feel like it's becoming a game again.
Vanilla/TBC/Wrath = WoW 1
Cata/MoP/WoD = WoW 2
Legion/BFA/??? = WoW 3