Constantine has never faired well being linked to a broader DCU even in the comics. I don’t particularly care if this gets made or not (it won’t, Elseworlds is just a fake banner for Reeves Batman) but is there a single good Constantine story remotely linked to the DC universe? All of Hellblazer ignores it for the most part and pretty much everything DC has done with him post-Vertigo is drastically inferior.
Still, Keanu is great and wants to make it. Great reason to make a movie even if it isn’t really the character.