I think I am going to try something a lot of people have talked about. Everquest. Never tried it...but a lot of people hold it up as an example of what could and should have happened to COH.
I've played EQ from its launch in '99 through today's expansions.
I tend to classify MMOs more from player attitudes and maturity than just what genre or year they originated. This is mainly for my own compatibility assessment, since the most important factor to me is that the population lives in the world and doesn't constantly pull me out of it (which unfortunately is also why most of you will have a hard time finding a suitable replacement MMO; the playerbase is just all wrong in almost every other MMO out there now).
EQ, DAoC, CoH, LOTRO and some other older games, in their prime were like that. A lot. CoH held onto that aesthetic better than all of them as they aged.
WoW was never really mature, it was too big. But it did have a ton more cooperation out of necessity early on than it does today.
Games today, even the old guard ones I mentioned, are solo-fests. They made grouping a dirty word, because not grouping pulls in more numbers (quantity not quality). And once a game lets soloers run amok, in no time you have way more mouth breathers and immature jerks than regular folks just wanting a MMO to commune with.
If you are going to try EQ, you have a choice between the live servers (I suggest Luclin/Stromm) or an EQEmu-based server (I suggest Project 99). P99 is the most faithful to how the first 3 expansions of EQ worked, which is its whole point of existence. The nice thing is that everyone on it realizes that.
You are forwarned: classic EQ like P99 is hard. Harder than any modern MMO would even try to be now. Which is actually its charm for those who love world simulation and challenges. You earn everything you get, every level, every item. But I guarantee you that you will actually REMEMBER getting those things down the road, since your adventures are imprinted on you due to their effort. Most people who played old EQ can still recall in detail their characters, zones, who did what, raids, and definitely dungeon crawls. In fact, CoH shares this aspect of memory to it also.
And the nice thing about classic EQ (and P99 in particular) is that everyone groups because you have to. It is a social and role-interdependence game, and you will meet people you don't know in real life who you will know throughout your character's life. You can get bufs, travel and services like corpse location/recovery from different classes, there's even a live market for that sort of thing. Sure you can do simple things solo early, and even some classes can get away with soloing for special reasons and places (I'm looking at you kiting bards). But by and large it is a role-based grouping game, with mobs and environments that would rip you in two in a second if you tried to step into them alone. Actually some will rip you up even with a full group at times. Even a severely twinked alt isn't a God and everyone dies from time to time.
No auction house, its all trade channel hawking and certain places known to be centers for trade (the East Commonlands tunnel mainly). No instances anywhere for anything, its all one big open world and the dungeons and raid content are also not instanced.
Everything from travel to dungeons to dying and recovering your gear puts the fear of bejesus into you, just like a really dangerous world should do. Mobs don't leash, they chase you until you die or cross a zoneline. If you die, you run back naked to get your gear wherever your body lies (yes, that includes deep in a dungeon; and yes, there are a couple options for locating or summoning your corpse to a safe spot in the zone but only with a necromancer to do it or bard to point it out). Death is something to avoid (xp loss and corpse recovery), unlike almost every other game that just puts a slight inconvenience on you for it.
There are always folks leveling new characters in P99, so finding groups early on isn't a problem. You may want to move to a starting city area that has more folks depending on what you roll (oh, and all races have their own home cities - complete ones - and local hunting areas). Even that is daunting and dangerous depending on how you need to get there (no flight paths; just boats, wizard/druids can portal you, or run).
Try it, if you like a dangerous challenge with no free lunch, you will love it.