Overwatch is only original in the sense that the IP didn't exist. There's a pretty decent (probably to be taken with a pinch of salt) article from Eurogamer which summarises an AMA and a few old articles and explains how OW came from "Titan" - https://www.eurogamer.net/articles/2017-03-14-blizzards-jeff...
GP's examples are more applicable to the question posed about late-stage pivots. IP is not the point. It's about having a particular business objective / product in development whose assets were redeployed to realize a different vision.
Overwatch came from the assets in development for the cancelled Project Titan MMO.
DoTA was built in War3's map editor.
Hearthstone used the IP from the Warcraft franchise.
Artifact used the IP from Dota 2.
Dota Auto Chess was built in Dota 2's map editor.
DotA spawned from Starcraft custom map Aeon of Strife using new mechanics from WC3, DotA:Allstars/DOTA 2 is a heavy revamp of the original map. Between WoW and Hearthstone there was WoW TCG, not a video game.
Overwatch is basically Blizzard's take on Team Fortress 2, which became a stand alone game from the old mod Team Fortress from Quake/QuakeWorld when Valve picked up the IP from the original creators John Cook and Robin Walker.
a better comparison would be how DOTA spawned from Warcratft 3, or how Hearthstone spawned from WOW.
a failed comparison would be how valve released Artifact off of DoTA2. but DOTA auto chess is now a thing. that may take off.