I suppose WoW has raid finder difficulty for people who don’t want or can’t commit to an organized group.
If you actually want solo though, it’s harder to imagine an implementation for that which actually keeps the content in some kind of recognizable state. Some older bosses can be solo’d because players have outgrown them in power, but even then not all of them are possible because of mechanics that aren’t possible without a group.
I guess they could make a version of the boss that they’ve stripped most of the mechanics so you can just see it for the story, but not sure how satisfying that would be.
There’s also the issue of making it so any spec could do it. Obviously tanks and some DPS would have an easy time, but a healer wouldn’t really do anything. Maybe there could be a bot mode that just fills your party with the roles you would need.
Personally, my dream MMO would have raids like they exist in WoW, but would also have solo and small group content that was made to be as challenging and rewarding as raiding, but designed with less people in mind for people who want to get that challenge without having to coordinate 9 or 24 other people.
One of my favorite times from back when I played WoW was early Cataclysm before they nerfed the heroic dungeons. It was really cool having 5 man content that I could do with whatever guild mates were on at the time which still required some coordination and put up a challenge. I remember spending like 1-2 hours on some of them early on and it was legitimately super fun trying to work out how to do the fights better. I haven’t really had an experience in a game like that since.