Something I wrote a bit ago. Hope it copies alright.
Disgaea MMO
Normal Disgaea classes apply
Any person can equip any item, obviously usefulness depends on class
Later classes can be unlocked like Final Fantasy 14 through reincarnation
Monster classes exist. There is no penalty for using the model of a low level monster if you like it.
You can reincarnate as the student/servant/whatever title of someone else. Only a low amount of students per master (2-3). Master can not reincarnate as the student of one of his students.
Can learn some skills from students. Only if you party with them though. Restrictions to skills apply either through limitations or locking you out of certain skills. You retain the learned skill if you lose your student or the student reincarnates as a new class.
Person classes can not learn 90% of monster skills. Monster classes can not learn 90% of person skills.
Need to add many more spells than original Disgaea games
Your guild is your faction
Every leveling area tier has two different possibilities. Area from 2-5 players and area for 6-10 players
Bonuses to XP (maybe others) for going in a party
Going solo is possible, but not as rewarding
Higher tier of equipment and such impossible without partying
Content focused on both dungeons and diving into your items (with the usual randomly generated maps) for better gear or improving your current gear.
Dungeons focuses more on giving the players a fun tour through specially crafted areas with lots of optional side content (a la Blackrock Depths, Stratholme from WoW).
Dungeons designed in such a way as to let the players take quick routes to their favorite bosses in 30 mins to 2 hours or spending the entire day clearing everything.
Both dungeons and item world can award you with better items.
Bosses occur in the item world as statisticians. Be careful because they're not alone!
You can of course skip floors if you wipe a lot on a floor, but you have to run for the exit!
You can transform any item to look like any other item, but only if you ranked up the item. Restrictions apply. (You want a Sword X to look like Staff Y. So you level up Staff Y. If you try to transform the look of Sword X into Staff Y it will have the stats of Sword X but will be a staff class weapon. So grinding lower level weapons to get their look is a time waster.
Guild master is named Overlord (or similar)
Overlord can create his own Castle (guild housing) in its own instance, can also designate other guild members to be architects
Dark Assembly is made out of vassals of the Supreme Overlord and you will have to convince them through quests/beating the shit out of everyone of them/bribes or just getting lucky into getting everything for your Castle like normal Disgaea.
Creating castles can be done like Minecraft/Terraria where you have to walk around or just have you flying around putting things down.
Overlord's Castle can have player housing, potential auction house, stores, item world portal, lounge area, etc.
Guild housing doubles as a guild dungeon, adding more content to the game with player created dungeons. This is also to make people put effort into their castles to show off how great their castle is.
Grind for materials to build the Overlord's Castle up to huge sizes over time. Dive into the building materials to upgrade them to higher tiers!
All PvP should have some equal rewards and/or incentive
World is not instanced, so world pvp will occur. Take cues from Hillsbrad, Stranglethorn, etc. to create areas where you have to choose one side to agree with (either guild or individual) and you become aligned with that side/town/whatever.
Battleground modes exist like capture the flag, KotH, kill the leader in a giant battle and so on.
Overlord's Castle can have a duel arena. Send your rivals duel challenges and show off your guild arena. Yours and their guild can spectate if they are in the castle (other guild formally invited to watch). Duels are scheduled by both players.
You will have to grind hard to get the highest level of PvP commendations. Only a few will ever reach the top which will have unique rewards.
Though every tier of PvP equipment will have unique rewards the best is obviously at the top.