Well maybe the concept could be to scare the most people. Scaring is made by possessing objects, items or even people. There might be a game time, that whoever has the most points obtained by scaring would be the winner.
There could also be one player or more which is called the Ghost Hunter. The Ghost Hunter can see these Ghosts, which are invisible to all other players except allies of the ghost and allies of the Ghost Hunter. The Ghost Hunter can use an ability to capture ghosts. The player owning the ghosts would be switched to observer camera first.
The only way of freeing the Ghosts captured are possessing the Ghost Hunter and using his ability called Release Ghosts.
The factor which you would use to detect how easy it is to possess a ghost is by their armor, or you could call it Ghost Resistance. The formula is Armor/100 * 100. That would be how much percent it takes to catch it. If the armor is zero, that means the character is Unable to Possess.
Ghost Hunters could also kill Ghosts. Killing Ghosts are made by the Ghost Hunters attack. Killing a Ghost is also a way of capturing them. To counter this, Ghosts may have an ability called Hide. This makes them invisible even to the Hunter. Both factions would have a safe spot. For Ghosts maybe an Invisible Castle of something. Ghost Hunters could be in a Tavern, where Ghosts may not enter.
Glad to help. If you need more, just tell me.