I don't like the
"It will be tranfered to your FC bank"

Because you'd need to block access to gil for everyone.
Even the FC owner

Lets say capped members, all are online, all do random duty finder with extra reward. They all finish their dungeon exact at the same microsecond all getting their reward gil ( ~2250 gil) at the same time.
Now all gil over the maximum goes into the gil bank. Which is Membercap x 1250 if no member has any gil on him but might also be 1250 to 2250 gil.
Now the Algorythm needs to check how much gil had each individual member had before the increase, add the new amount and save the new amount for each member.
This might sound easy enough to do some might say. But since every calculation is done for each FC member seperately this will ne a lot of equation to be solved at the same time.


And I won't even start how ridiculous this will be for FC housing.

Big Veto at THAT idea