Any brand new IP cannot, IMO, pop into existence and become a super ultra successful MMO (enough to usurp WoW/FF14).
WoW had Starcraft for years helping build a following for the company while also having Warcraft setting down lore and creating the world while also building a following. And Warcraft wasn’t exactly niche when it was an RTS. I mean, they had their cinematics advertising in movie theaters when that sort of thing was unheard of.
Final Fantasy 14 had more than 13 prior stand-alone games (when considering non-numbered titles like Mystic Quest, Tactics, Crystal Chronicles, Dissidia, Dirge of Cerberus, etc). So their fan base, aesthetic, and tropes have been established for a VERY long time (characters named Cid, Biggs, Wedge…Airships, Chocobos, Crystals, etc).
With that said, I think there is a lot of potential with Overwatch. You already have Tank/DPS/Healer delineated between the characters. There has already been word of a campaign mode coming…soon? That should help flesh out the world and lore of that IP. All of that are great first steps to eventually release an Overwatch MMO. Plus, if you want to make PvP a large portion of the game again then you already have that set-up right from the beginning.