You don't really give people a choice. It's impossible to make content that is equally difficult/tuned for different numbers (apart from scaling something from 500 to 5000 players ofc).
Just look at all the "solo" raid clears people have done with DK in WoW. Sure you can do them with a pally or w/e too, but it's more difficult. What happened? Everyone used their DK to farm raids.
Same thing happens with adding more difficulties. People who are "hardcore" don't get the choice, they'll have to do both (usually). Why would you reward one person the same way for doing something "easier" (which is hilarious since all my fract weapons came from 20+ and I have never even gotten one in 40+ gotta love that RNG...)
The only thing I can get behind is alternate means of obtaining stuff. They just have to be on the same level of difficulty. So instead of grinding w/e dungeon you want the gear from you could for instance get it from doing JP's. Sadly I've never seen anyone ask for the same difficulty of getting the gear from somewhere else. In WoW it was always "make it come from dungeons" which I have no problem with IF you make dungeons just as hard as raids so that would be X% (droprate)/elligible classes/6(pieces of armor). I'm in the same kind of boat as you but then with "guilds" so I surely understand where you're coming from
The reason why people would solo/duo dungeons isn't because they enjoy them more. In my case it's because it's usually easier (10m raiding vs 25man raiding)
I agree that they're nothing like an MMO that's because diablo isn't an MMO although if you follow some people's definition, it clearly is /snort.
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I don't really get how the world progresses in GW2 (or any other MMO), it's pretty static even with DEs. Any changes are simple patches which also can be gotten for single-player games. Same for persistent.
I think you would've loved GW if you played it in it's prime although it is instanced, having every town as a hub seems to be something you like to make the world feel alive.
I'm not judging you or your preference btw, I'm just trying to understand what actually gets you exited. From what I recall you like questing/map completion and hate dungeons/pvp. You also seem to want human interaction. The one thing I can't wrap my head around is if you want that interaction to be massive or if instanced would be fine too.