My 2-teams battleground (5vs5) based idea:
At the center of the map, will be a Fort, with an Emperium (fortified). The objective of the groups is to gain control of the Fort (destroying the Emperium) and remains with the Fort for 10 minutes. If the team remains with the Fort for 10 minutes, they win.
It's complicated to gain control of the Fort, the Emperium will cause massive damage around it (5x5 cells, attack 300~1000, 1 attack per second, only on enemies).
Each team has a Minor Fort, that create monsters to help. Created monsters has a view area of entire map and will attack the first enemy at sight (including the Emperium or enemy monsters).
Staying near of your Minor Fort, it will recover 10% of HP and SP per second and if someone dies, they will be warped to their Minor Fort and can't move for (BaseLevel / 3) seconds.
If someone go to the enemy's Minor Fort, they will receive massive damage (9999 per second).
Map: bat_c01 (a copy)
Example:
Emperium: Vit 5, Int 80, Def 50, MDEF 70, massive damage around 5x5 cells with an attack of 300~1000, 1 attack per second, 600.000 HP, holy, angel, large.
Spawned monsters: Zenorc sprite, Vit 1, Int 50, Str 50, Attack 200~500, 2 attacks per second, Bash, Heal ~1500 (heal allies, like Drosera), Meteor Assault, change target, holy, medium, angel, 5.800 HP.