1) Green DPS having to actually... Heal. (Found in every MMO except FFXIV)
2) The gear progression from BDO. The literal worst iteration of gear progression I have ever known in any game ever to have existed. First of all, all gear is low chance RNG drop from random mobs in the overworld. THEN when it drops you have to upgrade it. In order to do that, you first need to grind a bunch of materials that are low chance RNG drops from mobs in the overworld. THEN you have to pray to RNGeesus as you upgrade your item to +15 to then knock it up to rank I and if you fail any upgrade, you lose progression (Unless the item is an accessory, in which case it is destroyed completely). After getting it to rank I you then start to need upgraded materials to enhance it further (Made by combining 10x the low chance RNG drops you were previously using) and then continue upgrading to +15 and then to rank II. Rinse and repeat this until you reach rank V.
3) Ability customization from City of Heroes/Villains. Where each skill had up to 6 slots for enhancements which could modify anything from its damage, accuracy, cooldown, resistance (Damage Reduction), defence (Evasion chance), healing/regeneration etc. Including rare "Sets" of enhancements which would provide additional bonuses based on how many you have slotted into a particular skill. The end result was most of the classes could make builds that allowed them to solo raid bosses. Be it through capping out mitigation (80% resistance and 75% defence) while still having enough damage to bypass the Arch Enemies regeneration. Or in the case of Controllers/Dominators, being able to perma-CC said boss.
Oh, and you could recolour every single skill how you wished. Wanna throw neon green fireballs? Go for it. Zap people with black lightning bolts? Sure why not. Look like giant walking turd? Oh wait Stone Defence already did as baseline...
4) The Mastermind class from City of Villains. The way you worked is you only had a handful of support skills yourself... But you had a team of 6 summons to do your work for you. Be they a group of Thugs that throw molotovs and shoot people with uzi's, a bunch of Robots firing missiles and lasers, a bunch of Zombies that punch stuff for a billion damage, a group of Mercenaries that have assault rifles and rocket launchers and miniguns, a selection of Demons that can breathe fire or even a group of Ninjas who'd slice things up with Katanas and throw Shuriken and Caltrops at people.
5) Stat system from Anarchy Online. Stats are meaningful. You have like 10 pages worth of stats, with each one having a level of affinity based on your class and race going from Dark Blue (Low affinity, high cost to increase and low cap) to Green (High affinity, low cost to increase and high cap). With certain stats having "Trickle down" into other stats (I.e. You increase you Agility and various other stats like Run Speed, SMG, Dual Wield etc would also be increased slightly (Without going towards their cap). In addition, most equipment and skills didn't have level requirements, instead only having stat requirements. This meant that if another high level player could buff you, you could equip items and use skills that were way stronger than your level (For example, when my level 20 Engineer got buffed by my brother to twink on a level 140 Robot minion... So much higher level than me I couldn't personaly see it as it was simply Level ???. Thus letting me solo missions much higher level than me by just putting my deathbot on Hunt and going AFK while he murders the entire mission... Unless it was those damn OP robot dogs. F those robot dogs.)
The sheer level of customization in that game was incredible. Due to so much flexibility with stats. A lot of it was viable too it was simply a matter of how hard you worked on it and how good gear you got to support your dumbass choice of using for example, weapons that are Dark Blue affinity (Which, at one point I believe was actually a potential max DPS strat for Soldiers, to dual wield Pistols/SMG's instead of their natural affinity of Assault Rifles)
6) Astellia Online's approach to MSQ related dungeons. In that game, when you have to do a dungeon for the MSQ, you run it solo. No NPC help (Other than your Astells). No parties allowed. Alone. In an easy version designed to be done solo. Purely for story. Then, 10 levels later, you unlock the farmable versions. One which is solo but still gives loot and is more challenging than the MSQ version (Of course, this game has Astells, "Pokemon" that can fill in a role for you like Tank or Healer or DPS to aid you) and one with is multiplayer and gives more loot than the solo version (Also, loot is individual. So when you open the chest after a boss, instead of it dropping 1-2 items that you roll on, EVERYONE gets like 5-6 random items from the pool)
7) Blade and Soul's dojo that tells you how to play your class. Including a number of demonstrations where you have to do specific combos of skills and/or have to not stand in the giant yellow telegraph.
8) Guild Wars 2's open world events (Aka FATES) which not only don't take a million years to complete, but actually give nice rewards like relevant experience and gear. (Also featured in Rift's err... Rifts and Warhammer Online's Public Events)