The above kind of already exists for dolmens. If you're running a dolmen by yourself, you get fewer enemies spawning and a gentler final boss. Meanwhile, ten people running around means beautiful chaos, larger swarms, and a meaner final foe.
I ask because, in truth, it can get hard to find people to do things with. This isn't me doing a "server population is down. WHERE IS EVERYONE?!" alarmist thing, mind you. I don't mean it that way. It's just... there are a lot of zones and the player population is usually spread pretty thin across them.
I can almost never find anyone to do Harrowstorms with, outside of seasonal events in Western Skyrim. I never see anyone doing Bastion Nymics anymore, and you want at least three people for those. The more difficult world bosses in the newer zones? Most aren't made to be solo'd, and you're usually out of luck unless a charitable soul sighs and says "fine." If you aren't pushing yourself those first two months of an expansion, you're going to be having a bad time real fast. That public group dungeon boss in Silorn NEEDS a dedicated tank to be anything other than a kiting nightmare, and how many people are running Silorn six months later? Almost no one, outside of a few people getting alts through it for the skill point and skyshard.
I like the community note of experimenting with group content like 2014 Craglorn again, in theory. The talk about adding gear-chases (back in my day, we called them gear treadmills) makes me cringe because I don't want to have to grind for Perfect CP170 kit and part of the appeal of this game was you NOT having to do that (I quit WoW because of treadmill fatigue)... but again, in theory, could be fun. You know, as long as you get it all done while people are still doing it!
Because if you don't, it'll be like the Bastion Nymic. Dead content that you'll have to beg guildies to do with you for the achievements.
So yeah, it'd be nice to see a dynamic difficulty scaling system introduced, maybe after X number of months. If you're fighting the things alone, we'll tone the mechanics and attacks down a bit to enable you to actually get it done... but you're going to get green items instead of purple items and can kiss some achievements good bye. A worthy trade-off to actually see the content after the throngs leave for greener, newer pastures.
Just my two cents.