I just have a few ideas that I think would be fun to play online.
I think pvp makes a game more interesting. I was thinking about an MMO where you start off in a guild that is correlated to the class you join. Have the guilds associate with certain things you can do in game. Like if you chose a paladin guild, you are liked within a city and have abilities which are good against effects like some sort of darkness ability.
If you choose assassin or thief, you are hated in some cities and are more likely to be attacked by guards.
Have both guilds types (good and bad) have their own network and city structures. Like paladins can teleport to a holy city and necromancers to a dark city.
Have neutral classes such as soldiers and some type of healer. Also, I was thinking about places where players in high level would be interested in. Like a place called "field of power" that contains resources that players can use to craft things, create new spells, catch exotic creatures and so on.
Have interesting areas and interesting quests. Like if you are a chemist, you can get a quest to find an exotic flower, bird or whatever. This quest takes you through dungeons with Indiana Jones type traps, puzzles and different types of paths. Paths like a hole in a tree which leads to an isolated area etc.
Make certain trees climbable where you can fight stuff up on it. Trees so huge they can be considered dungeons.
Make group ambushing possible by giving stuns and bonuses to surprise attacks and making use of trap makers. Have an option that becomes available when you are well hidden.
Very high cliffs you can fall off of.
Make environment play a role. Like if you stepped into a bush of thorns, you have damage taken off over time or if you step on a tar pit, your movement will be slowed. If you fight on ice, you will miss a lot.
Have certain classes affect the environment in their own ways. Like mages can affect the mood of the environment or holy priests make their environment lighter and necromancers make the environment darker.
Mages cause it to rain if they use certain elemental abilities a lot in a certain area. Have rain and windstorms affect the combat effectiveness of players. Give the ability to create randomized spells and capabilities. Like if a player prefers to play one style and he is a soldier, then he gains a unique ability according to his style. This all randomized but balanced at the same time. Like coding it to be this one ability cannot be with this other ability or this one ability cannot be used more than twice an hour depending on the class of the ability.
Make raids to cities or opposing factions beneficial. Add player leaders such as guild leaders being able to declare war on other factions and it being legal in certain cities.
Have guild duels be possible and beneficial. Have the ability to mass produce items to outfit newbie soldiers conscripted for guild warfare. Have it so guilds can pay recruits for war per 20 minutes or hours depending on their abilities or whatever the guild lead wants to pay.
Have a general pay per hour system if you want to hire other players for anything. Give benefits for environment in terms of combat.
Like fighting from bushes gives you increased evade, fighting facing the sun debuffs you with missing more often and taking critical damage at a higher percent. Fighting in a ditch if you are range adds defense if they are not in that ditch with you.
Have technology that is affected by the environment. Like you can make a glider and dive off a cliff. Make crafting more useful such as engineers can make submarines or something like that. Chemists have chemicals that allow you to ignore damage from small environment factors like bushes and reduce critical strike damage taken from fighting facing the sun.
Just a few things I think would be fun in an MMO.