If you are lvl 20, and go back to a lvl 10 area to catch up with the campaign, presumably it wouldnt take that long time before you got back to areas within your scaling range.
Could also make the ranges somewhat bigger. Like, first zone; 1-10, next zone 5-15 etc.
If someone lvls to 39 before doing the campaign? Well, maybe dont do that! 
Or just get that last lvl which makes the whole thing scale up to 40.
I’m not strongly against the game scaling up to your lvl, as long as it doesn’t scale down to your lvl. But it feels more interesting if it doesn’t.
Also, since I think there should be NO difficulty slider while lvling either, outlvling content might be the only way people have to make the game easier. You take that away from people, if it always scales to your lvl.
I guess you could do what some games have done, and let enemies only scale up to -5 (or whatever) of your lvl, if they get below that. So you can outlvl enemies somewhat, but not to the point where they become meaningless, or cant drop useful items anymore.
Btw, speaking of scaling. One thing I have always enjoyed in some RPGs and MMOs, is when you run around in like a lvl 10 zone, and a big lvl 60 boss is walking around. You cant do anything about it, but you know that one day, you can come back and destroy it. I would like to have those kinds of enemies, areas etc. outside any kind of general scaling system (and no, such enemies of course shouldn’t randomly attack and oneshot you either Might only aggro if you attack them or whatever).
Yeah maybe. But I have some issues with that. What if you have done a tier 8 key dungeon, and then respec to a weaker spec (for whatever reason. Maybe Blizzard nerfed your other build, maybe you were just bored), are you still bound to see only tier 7-9?
Considering how large D4s world is supposed to be, I think random spawns could be fine.
It should of course be reasonably easy to see which tier the activities are, before you enter, so you dont risk getting randomly oneshot.
As long as the scaling isnt stupid, even as a character who can do tier 10 well, it shouldn’t feel like a total waste to do a tier 5 boss. Lower droprates sure, but also faster kill speed. Might not be the most efficient thing to do, but it might still be more efficient than ignoring a world boss right in front of you, and travel to the other end of the world (literally) for another challenge.
