I would ask that you create a mechanic that:
Because?
Who will play the version in the xbox will have all the game until FG.
I wanted the experience of playing the base game, after playing each DLC individually.
In addition, this will make it easier for other players to get a set of a specific level.
I 2nd this, I have trouble farming low level items because most of my play time ends up being at max level. I have a serious lack of sub level 50 blues and sub 75ish purples. It’s almost impossible to get a “self played” low level set because you out level the drops so quickly. It’d be nice to stop leveling and just play the game at a certain level, it would create some additional challenges as well.
I would love to stop leveling on elite and be able to play through all the content with all the rogue dungeons without maxing out or near maxing before ultimate.
I would also love to be able to collect set below the maximum level …
And I imagine it’s easy to add an NPC
that by talking to it, it will lock or unlock the experience gain.
Please CREATE, does this ?! 
+1 to this.
As a player with a somewhat unrealistic goal of collecting one of every epic and legendary item, being able to lock the XP gain would allow me to target a range of items to farm in SR/Crucible/MC (Like ~level 14 epics or ~level 50 legendaries)
In World of Warcraft, we call the process of locking character on a level to make certain content of twinks. There are even guilds just for that.
Please CREATE, does this ?!
+1 I love to grind in some low areas, but then you reach a too high level and got nothing. 
Well, you could cheat a bit and make a backup of your low-level character, farm 'til you level too high, put everything worthwile in your stash and restore the lower-level backup of your character again…
How about an inverted XP pot that gives -100% experience for an hour. I’d call it Potion of Obscurity