Really, I could see them launching their own engine. Think of all the studios and talent they have now. They have the engines behind Halo, CoD, WoW, Overwatch. Could build an Unreal competitor.
id Software is not the company it once was, but they still make engines. idTech 7 is their last one, powering Doom Eternal.
idTech would be great for Halo and Call of Duty. But it isn't great for The Elder Scrolls, Starcraft, Gears, and many different games Microsoft Studios are working on. EA already tried to make every studio to use Frostbite for every single game and ended up with disasters like Dragon Age Inquisition and Mass Effect Andromeda.