Okay richy, and anyone else interested, I'm going to go very indepth in my recommendations for you since you're not sure. I'll try to hit those few things you did specify to the best of my knowledge XD
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World of Warcraft - Better yet why not play a clone game for free that has a playerbase that will outlive WoW?
https://www.youtube.com/watch?v=CdqFYBeTUac
Runes of Magic holds the record for most spent to make an MMO (2.2billion I do believe?) and has expanded greatly beyond that. The game is a literal clone of World of Warcraft with improved PvP and a Multi-class system. With a total of 66 different possibilities of class mixing the game has become a staple of customization. As well as stealing WoW's players.
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Looking for blood and amazing fantasy combat? Action Combat is an interesting genre of MMO in which you actually have real fighting physics and need to aim your swings and spells. It's no longer a point and click world! Nothing captures this better than one of the first games to have this feature.
https://www.youtube.com/watch?v=tM2yfu_IqV4
Vindictus is a top rated MMO that has been a favorite of gamers since it's release. Although it has a limited number of gender locked characters... each one has a completely different play style and two different very varying weapon trees. I believe the current numbers to pick from is 7. The game has a very dark cinematic feel with loads of blood and fast paced combat. It's a TRUE MMO, without social interaction you won't get far. Also having to actually aim your attacks makes this a game where skill is required. Although, I haven't tried the PvP, I hear it's amazing! (Plus playing this, I'm more than willing to meet you in game when I don't have class! This has been a favorite of mine for years!)
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Science Fiction? EVE is a good game for that, but if you want some nostalgia I have to recommend you go a little advanced in route and play Japan's number 1 MMO of the famous science fiction series Phantasy Star.
https://www.youtube.com/watch?v=az7gD1wCguQ
Phantasy Star Online 2 is an extremely unique game. Pick a race from the classics of Phantasy Star, as well as a new corrupted race, enjoy the vast character customization, and jump into a universe that would be all too familiar with anyone who played Phantasy Star on the PlayStation 1 or Phantasy Star Online on Dreamcast. With the ability to change your main class in the lobby anytime and even subclass to mix in skill and weapons it's a game that will never tire. Play one mission as a Third Person Slasher (Semi-Action Combat like Vindictus), play it as a Third Person Shooter, or play it from the back as a true DPS force user! The game is in it's Episode 3 chapter and has recently made it so ALL classes are unlocked right from the start. Great time to join the game.
However I warn, although this is Japan's #1 MMO in terms of income for Sega and population... but it is also full of Anime cross-overs and other Japanese culture... the game is Region locked. It will require signing up for a Sega.jp account, getting the English patch, and depending on your service provider using a VPN to play. Due note, although it's technically bannable, Sega.jp has announced they are fine with American players as long as they don't harass the native players. Ship 2 is FULL of English players, JP players have called it the "Devil Ship".
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Mature? In-Depth? I've not personally played it yet... but my friend has and I've enjoyed researching the game.
https://www.youtube.com/watch?v=SNFSw107MQs
Black Desert is the future of MMOs. Everyone is excited for this revolutionizing world in which you come as close as possible to that of .//hack or Sword Art Online. It's a simulated world in which you can do anything, even being communities with the uniquely functional real estate system. This will be a game where you can live life as a merchant or a monster hunter. Plus the game is one of the most visually appealing that will be releasing soon. It also follows the amazing Action Combat system that all MMOs are switching too.
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Horror is the hardest... The best I can come up with for MMO.. and it's not even a full MMO is DayZ
https://www.youtube.com/watch?v=waFpwpDHY3c
DayZ is an interesting mmo in that it includes permadeath. That makes this a game of mass hysteria and paranoia. Once you've invested an amount of time into a character you will start to fear going into towns, not only because of the zombie hordes, but of other players possibly being player hunters (aka Bandits). The game is the best survival game out there and although the MMO is limited depending on which version you play, the game still has the social interaction enough for me to consider social.
Your options however for this vary,
DayZ Standalone - The standalone version of DayZ has released. It's in Alpha, but it's good. The engine is strong, the graphics are better, and overall it WILL be better than the mod when the dev team finally gets off their asses. Biggest cons are the lack of content and the max 40 player servers.
DayZ Mod - This version is maxed out in possible content. Things from building bases to vehicles that are missing in the Standalone. Not only that but there is a program which allows you to MOD THE MOD! Adding even more content and game modes... overall for playability the mod is far better, for now.
Both of these however, unlike all others of these recommendations, cost money. DayZ Standalone I do believe is 30$ on Steam. For DayZ Mod you'd need to buy ArmA 2: Operation Arrowhead (which is an Army simulation mmo also, so it's kinda two birds one stone if you're into army simulation).
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Lastly, it doesn't exactly fit what you're looking for, but it is my personal interest at the moment. This game has been in-development for at least 2 years (most likely at least 4 since 2 years ago was just the last trailer), but the style of this game and it's Action Combat system looks very interesting. I will CERTAINLY be getting it once it drops.
https://www.youtube.com/watch?v=kCTMoMueRr8
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Those are my recommendations... All of which are extremely active and have mature players. You'll find less mature players on point and click games (like Runes of Magic sadly), as they have an easier time just mindlessly playing the game. Action Combat is honestly becoming a standard in modern MMOs thanks to Vindictus... and more mature players seem to stick to those nowadays.
not really an RPG.. more like a warcraft type thing.
. I guess I'm still checking it out though. Hope springs eternal. =)