While you could argue that Kayle fills the role, she isn't exactly active in the plot right now, and one could still have another character who more closely fills the niche. After all, HotS has both Whitemane and Imperius as the zealous "Light is Not Good" types. I also wouldn't personally label Kayle directly as an Inquisitor so much as a Seraph.
Seriously, I'd love for League to get a character like Whitemane, complete with her "Ohohoho" evil laugh. It'd be great to add in a character who takes advantage of the current state of Demacia to further their station by rounding up and executing mages (and those suspected of aiding them). We could have a great new villain to add to Demacia's roster.