Seriously, go play Oblivion. Experience the horrors of level scaling on exploration.
Just "monsters" isn't enough, I can exactly say which one. In EVERY SINGLE DUNGEON. Unless I go to level for the next monsters, who then again are also in EVERY SINGLE DUNGEON.
Fun, there ain't none.
You can obliterate all challenge by getting super-gear at lvl 1 (heck, the best strategy and advised tactic is staying 1 ALL GAME. Any RPG where the experts advise you to do that... I have no words). Of course, by the time you get 20 every single enemy wears super-gear too, and any and all "proudness" of getting good gear is gone seeing it being as common as rocks or trees.
"This armor is rare"
"Really? I saw a highwaymen asking me 5 gold wear the whole set, worth 9000 gold. Rare indeed."
It may be good for a hack&slash like Diablo, but seriously, what exploration or plot or believe of a world is there in Diablo. It's all about loot. That's, fortunately, not the case here. And I am glad for it.
Yes, a bad designer does that. And I am glad the designers of D:OS don't. Despite the apparent claims we now get they should get this, and the game's too hard and should simply allow you to go anywhere you want and addapt itself to that. I am very VERY glad they decide to say 'no' to that.
Okay, here's you anwer;
* Expansion will raise level cap (there's a shocker).
* Mods will add appropriate content for the environment it's added to (if it's good that is). Unless someone intentially wants to make a lvl 20 encounter area in Cyseal. More power to them. They got all the freedom they want designing. I absolutely see no issue there myself. Why would there be issues?
(Not to mention total conversions do indeed generally force you to restart. It doesn't make much sense bringing your full character into a total conversion mod, most of the times. I'm sure there will be 'arena' mods that will allow your character to test it's maxxed out metal)
Maybe if you actually raise concerns I can answer them. Because as far as I see, there are none, just like with the current fixed enemies approach. What's wrong?