Completing the run on time will have the same results it currently does in 7.2: you’ll get a new Keystone for a random dungeon that’s 1-3 levels higher than the one you just completed, based on how quickly you finished.
If you complete the dungeon but don’t make the timer, you’ll get a Keystone for a random dungeon that’s 1 level lower than the one you just used.
If you don’t complete the dungeon at all, you’ll have a Keystone for the same dungeon that’s one level lower than the one you just used.