Since a 40 cal model was submitted for the MHS, and the 9mm and 40 cal use the same frames ( with no model markings on the frame) it is probable you can run a Glock 23 upper just fine on a 19x frame. I have been told to run a gen 4 slide on a 19x frame you just need to put on a gen 5 slide cover plate. I don’t have a 19x yet to try it but somebody has probably done it already. I could see the 23x being released probably before a gen 5 22. ( edited end cap)
Will a Gen 4 slide even function on a gen5 frame??
It's possible we could see a 23X but with .40 dwindled popularity it's slim.