NOTE: this is entirely from a game-mod perspective, as to vanilla, I am not sure you should expect any change at all in this regard.
There are arrays for a whole lot of things in the game that are indexed on “Normal/Elite/Ultimate”. But there is one field that has a single value and that is the final boss dbr, presently Log. So within these confines, you really cannot shift the transition from Normal->Elite or Elite->Ultimate.
You can just remove Ultimate and yet you cannot. If a modder changes the game playthough model, and really wants 2 instead of 3, you cannot prevent the player from entering the 3rd. You could make it identical to the 2nd, but it will just exist as that is the way parts of the game work that are not moddable.
However, through the quest system, you could grant the player all the rifts in the world to speed up traversing the world. You can, using the editor, add elements that would allow additional rifts to further speed up the traversal process.
But then, the placement of where in this process someone reaches max-level becomes very much a point of opinion and quite varied are those opinions.
IMO, the speed run mode should be open in all 3 difficulties. Normal, you use it a bit less in the beginning, but by the time you have killed the Warden, you are ready to just hit each boss/quest/shrine and move on to the dlc and challenge-areas. I would like to see the difficulty ramp up a lot after that. I would like to see the game get really hard and you would find yourself speed running less in the second playthrough.
For me, at the end of all the dlc and challenge-areas on the 2nd playthrough, I would like to reach the level cap and just stop with that toon. The playthough 3 would still exist, but I would ding my last level right at the end of the second. For people that want to have another resistance penalty and just farm loot, I would just leave Ultimate there (as it will not go away anyway) but design the game-mod so that you have finished it at the end of Elite. In addition, I still get sad after level 50 with only 2 skills points. So I would rescale the higher leveling process to be slower but maintain 3 skill points per level. This would lead to a reduced max level. This would lead to a change in loot level requirements as well. Once log is dead, you should be at a level that makes legendaries drop and be usable while running the dlc and challenge-areas. But of course to enforce this for all players, you would have to transition XP gain to the quest system instead of the kills system. As someone that does not speed run normal will be far higher in level at the dlc. See how messy this is all getting?
(and yes, I did write that all out as this is what Grimmest might become and I am mildly curious of the feelings of others as to if this is appropriate)