I would be surprised if they take it to that level, but maybe! They seem to have blended maps and rifts together and of course:
Mythic+ dungeons from WoW, which I also enjoy.
My favorite thing about the keystones is that the challenge of the dungeon is static, so I won’t always be forced to beat a 15 minute clock. I can make the build I want and then research dungeons that it might be effective at clearing. Good. Stuff.
I don’t mind at all if they have a hard cap in the Keyed Dungeons, as long as its like you say - quite difficult for the best of the best. I doubt the shadow of D3s shoddy scaling as time went on will manifest in Diablo 4, for various reasons. Though, I can see the level of Dungeon rising if they announce an expansion that raises level cap or something like that, which seems only natural.
Then again, I also don’t mind if its completely open ended, just as long as power creep isn’t introduced carelessly because “GR is infinite, so power creep doesn’t matter” type of logic.