A lot of people complain about overworld content being too easy, and companions only stand to make overworld content even easier. The current solution to make overworld content harder is to just intentionally gimp yourself, and in a game about making your own build, that's just not fun. I've seen people suggest a 'hard' overworld instance, but that just runs the risk of splitting the playerbase, which just isn't fun to play in, plus, that sort of thing could potentially create abuses by having two of a zone. Plus, what is hard for one person isnt necessarily hard for someone else.

What if ZOS introduced a set of mementos to the game, that increased overworld difficulty by lowering your own damage and health, and increased the damage you receive from enemies in the overworld/from quests. ZOS could have different mementos for different 'difficulties'. ZOS could even introduce rewards/incentives. And this avoids a lot of potential drama/ issues with other solutions (though there is still the problem of other players blazing through without activating increased difficulty and killing your enemies while you are fighting them).