Since no genre was even specified, these responses will be all over the place. Perhaps too chaotic.

1) Have the entire world run on AI. When the players jump in, the take over the role of an existing AI NPC, making them their own. This allows for a world where you never know if someone is real or not, unless they chat with you, or they are on your friends list and are flagged. You can join a team waiting to go on a quest, and not know if your team mates are AI or real. If someone drops out, their character continues.

2) Obvious list: Head to Head competition, Team vs Team competition, Leader Boards, chat

3) Buff's (DND mini game on facebook, uses the concept that players currently online (and on friends lists) can 'buff' each other, or give temporary boosts)

4) Team Strategy. I.E. Maze Game. Each player is on their own, but feeding each other information about the environment.

5) Team Points. What ever game, each player gets a free amount of points per amount of time. They get a buffer. They can spend these points on extra magic, or dropping things into the game to help them. If one player doesn't do anything with their points for a while, and it reaches say 100, immediately 25 points go to a Team Pool. and the player goes to 75. This allows team members to agree to hold off on useing team magic until they can afford a larger spell or something, or if a player goes off line while the team is still active, that they are still contributing some how.

6) Face Mapping. Have a 3D character, Have a video camera. Setup an app that automatically uploads a image of your face (automatically cleaned of background) ever 500ms or so. HAve the game keep mapping that image as the face of your 3D character online. the closer a character is, the more often it will update. That way you can see the real expressions, etc...

6b) same thing, but have the app map the expression, and simple change an expression on an existing face. I think it would be interesting to see the expression change to confident, alarmed, distracted, excited, etc... mid game play. Much of real life game play happens in the facial expression and body signals. It would be interesting to have this carry over into the game.

7) Voice controlled AI Team members. Have an open chat between members on a team. Include AI members of the team. Include voice recognition. When a player speaks the name of an AI team member, they can follow it with certain commands or suggestions, and the AI team player will comply. (depending on your rank in the team, and what other requests were sent to them.)

8) Interactive Ghost Review. Similar to Starcrafts replay of entire games with full clarity, Having a replay, except for a 3D RPG style, where you can walk around as a "ghost" character in the world, with another party member, and replay/rewind etc.. Presented as magic and a form of training. Like an AI instructor who teaches you battle strategy. But with multiple players walking around. Even allow them to be in different times. By showing all "ghosts" as transparent, and if they are not in the same moment as you, it shows as a bar over their head indicating forward or backward, and you can click them to see what they see.

- It puts an interesting time element into the game.

9) Commander and Soldier

- Split games, where one person gets a complete battlefield perspective, and gives orders to other players. Perhaps a couple key roles of different play style.

- or Commander has over View of the whole battle field, and can controls several squads of AI infantry. Then each other player leads a specialist squad, such as demolitions,

10) Driver and Gunner

- Split game again, where one player focuses on maneuvering and avoidance while another focuses on shooting.

I love being able to create galleries of collectibles or gear/clothes/weapons on mannequins, and being able to look at others' collections. This is a feature that combines really naturally with player houses if a game has them. I've never yet seen a game that let me mount the heads of my non-human kills on plaques and decorate my house with them, but that would be fun. Games with pets could also display those as a collection, like a fishtank or a meadow or stable full of pets. (Games with pets do sometimes allow people to breed with friends pets or trade pets, but those often end up being mis-features, a bad idea because they destroy the motivation for the player to work their way up the game's genetic tree by themself. So that's a discommendation rather than a recommendation.)

Awarded titles are fun - I like the system where earned titles and ranks are available in a list from which a player can choose up to three by which to identify themself. Like, the same player could choose to be Joe, Master Fisherman, Pet Breeder Lvl. 47, Rescuer of Maidens, or they could choose to be Joe, Chicken Kicker, Winner of Gulden Eating Contest, Slayer of Beetles.

Awarded titles are fun

Bah, I can't believe I forgot that one, I definitely second the use of Awards.

Perhaps another concept, but still to do with awards, is the idea that Guilds can award Guild Members for activities. I.e. Depending on the Guild's size, it receives up to X number of Gift Awards to give out to selected members. Anyone but the Guild Owner can receive it. You could allow the guild to title the awards as well, Such as [Guildname] [award title]. And perhaps even apply certain bonuses to it, like +1 to X unit Armor type, or something. This could be part of the reason to join guilds as well. For the general support of Guild level upgrades and what not, but also for special titles received while there.

Perhaps guilds can have councils where they propose things, like The next Guild purchase should be X skill increase. and the counsel members can either agree to it, not agree, or mark it for Guild Vote where everyone in the guild gets to agree. Then put a time limit on it, so if people don't vote with in 48 hours, then it tallies only those who have voted. And perhaps enable a faster vote for emergency responses that are enabled by the Guild Leader.

For that matter Sim City from the Sim People perspective, except that its more about how the population votes. I.e. you control the vote of what happens at your house, I.e. you get money to spend and you can spend on cool stuff. But, your neighborhood also gets to vote on stuff, where you vote side by side with other active players in a certain region. Split things out in certain ways that it could do more good in isolated ways than being distributed lightly. For instance, a police station, fire station and park district each receive a small boost to funds for minor productivity improvements. OR, you could focus that all on the park fire department and get a new fire truck, reducing insurance costs, etc... Your vote gets tallied with all other players in your neighborhood.

Then of course you have city level. It has less direct effect on you, but you can decide on it. I.e. Allow a Casino in your city, and people receive tax incentives, but crime will rise. or implement better building standards in the city, reducing fire risk, but it raises building costs. Essentially turn it into a game that people can join a new city that starts every week, and though out the week they keep checking back for new law votes. As a team effort, everyone gets the opportunity to vote, if they log in often. and they can all see the effects it has. the longer people are logged in and doing things, the more money they get. and the city and the players get a score at the end of the week. The city gets ranked against previous cities, and within the city itself individuals get ranked by how well they did in improving things, and going with the options.

Then, the types of things that happen keep changing with every pass, so the events and options keep changing. IT doesn't need to be a balanced game, as it keeps changing all the time.

Perhaps there are elected officials in the game as well. (I.e. the previous games top of the leader board, who weren't officials in the last few cities) The elected officials can focus on things, like identifying where the new school being voted on will go, or identifying where the parks in the new neighborhood will go. Proximity and setups will also factor into the success of the game, but the amount that players are logged in at various locations in the city will define more things about the success. For instance, if the majority of the people that are logged in are near the police station for that neighborhood, then the police will do a better job. If the people are farther away, they will do worse.

I'm done rambling now, but I believe that does bring up a bunch of ways that players can be interactive in direct and indirect ways.

How about time travel. I haven't seen that done before. Here is an idea that has been circulating in my mind for a few months, where different players exist in the same spot on the map, but in different times, and the times affect each other.

For instance, X number of players agree to join a match. A city is generated, along with a random villain/evil plot. Your characters can travel in time, and differently from each other. But then a player in an earlier time triggers a bomb while investigating. They might die or get injured, but in the future, if another player is at that location, suddenly the wall disappears, or they fall into a crater.

Going to the future will give you additional details about what happened and where, but it is more dangerous, because you are tied to the past, and any change that happens in the past could effect you. The farther forward you are, the more things like cars suddenly appear and disappear (as their routes changed and you get butterfly effects from player choices) Your objective is to work as a team to investigate an event, figure out clues in various parts of time and on the original starting time, catch and stop the person before its to late. (I.e. bomb goes off) The players could even chat between each other, and say I'm about to do something in front of the grocery store. and then other players no that the grocery store is risky. to be crossing the street near. The game would get perhaps an hour to resolve it. and your time travelers start one hour prior to the event, and other wise can move in 15 minutes increments up to 1 hour past.this gives you at least 2 or 3 times to see the event occur and get different perspectives. Can you resolve it before time runs out?

the perspective of Camping can also occur, where all the players agree to let time slip forward a few minutes. I.e. they found a bomb setup in the future, and in the past its not there yet, so they don't know who did it. They agree to let time slip until the person shows up, so they can catch them in the act.

More difficult settings could be run, such as giving the villain henchmen, or Plan B. I.e. Since they saw you watching one spot, they decided not to place the bomb there while everyone waited. then someone in the future suddenly sees the bomb dissapear, and then they know the villian is in sight. or perhaps just one of the henchmen.