Would strongly recommend Guild Wars 2 right now actually. Its a solid, fun game in its own right, nice combat options to explore, nice progression/build/class system that doesn't want to go too deep but does open tons of options per class, and most importantly: NO tiered progression as you have it in most other MMOs.
Level progression is open for a large part. So if you step into this, you can really forge your own story, be your own character exploring your own world in your own way. No need to step into grinds either, unless you really want something specific, which then becomes a goal in its own right. You choose those goals, its not like they made you an endless path to walk down to 'git gud'. Your character level even gets matched to the area you're in.
When you hit max level, you can start exploring Fractals and other high level dungeons and raids to get your 'tiered' progression fix for a very minor stat bonus. That's all the gear has for you in tiers - you can simply craft your 'best' pieces on the spot with the right professions. Other than that, gear stats are simply differentiated by finding the perfect balance between stats and items that contain combinations of those stats. You can slot some gems in them to amplify that further but even that is not layered into infinity. The game also has a WvWvW and several PvP systems. The PvP is REALLY good, the game was primarily built around it.
If you're an MMO-vet and have seen it all, Guild Wars 2 offers the perfect cocktail of 'any and all activities' you would want, in a fun, well polished and highly dynamic combat system. You don't need several hundred hours to be king of the hill again, either. And its not a money sink either, there is a shop, but you really just want to not use it for anything other than cosmetic stuff. Its also, very importantly: NOT like WoW where you spend the first week or two catching up and then catapulting yourself to the casual grind everybody is doing ad infinitum. Been there done that, GW2 is more fun.
The quest content is immense in this game. You can solo the whole thing except for dungeons, and solo-ing the later world areas is a real challenge and contains tons of stuff to just 'do'... jumping puzzles for example. Maps also have lots of verticality in them, and some areas are really hard to get to.
Beyond the good old classics (WoW is still solid don't get me wrong... its just... WoW, and everything's already been done before, and no, you won't get the vanilla vibe again, I tried really hard
) the MMO landscape is pretty much dead right now. The innovation has been done and reached its climax, the vast majority is just shameless money pit/copy material, and some great names have remained. MMO development is bottom-ladder game development now, not too far from what BDO is, grindy money pits with no guarantees.
Matter of fact... I'm triggered now. Installing...
Another notable 'explore the world and have fun' MMO I could probably recommend if it wasn't for the price tag which is far too high: Elder Scrolls Online. I played this since beta and on-and off with later expansions... but yeah. Had a rocky start and is still rocky, but the world on offer is amazing, until it starts feeling samey, which is when you cancel your sub and forget it fast. But while it lasts.. yeah. Worth. But where I would still today say 'invest a bit in GW2'... I could not say the same for Elder Scrolls Online. Bethesda's been having a laugh with its customers and it doesn't deserve a reward IMO. (They basically lifted a middle finger to paying customers and now we're all paying, you practically have to pay sub AND every content update while it was supposed to be B2P).
Dunno about active guilds but you can probably solo the whole game(including expansions) if you wanted to with heroes.
Another decent semi-mmo game you can try out is Monster Hunter World + iceborne, though its not MMORPG per say but it has elements of MMO where you grind out materials to make better armor and weapons. The game visually looks like a "hack n slash" but its not, there is skill and patience involve when fighting giant monsters. The game is pretty much a constant boss fight with some story and some side quest you can do to acquire things, but the fun side of the game is being challenged by all the wide variety of giant monsters.
Best time to play MH now as they have finished all the DLC monsters and right now they have all the events active for players.
You can join random people rooms or if you're stuck on a assignment you can send SOS and hope for random people to join and help you.
If we're going into 'instanced online', Warframe is another one of those wannabe MMOs that's still a great place to visit.