Basically an idea from a few games I played. Short & Simple.

72 Players!!!
3 Grand Company castles/towers/fortress/etc...

Players must fortify their Grand companies fortress and while trying to capture the enemies Flag at their fortress. Players try to breach and fight their way to flag!
It could be a fun Capture the flag with Traps, A-Rank boss spawns to protect fortress or cause havoc, and different areas to attack from.

I'm too sleepy to go unto deep detail but that's just the short version of it.