Dessire
Regent
It could be something similar to seed vault
It could be the autoaid like at level 80 you can use the auto aid 3 times per collection + a passive bonus like FPs or attack.
It could be extra goods, supplies , gold , FPs, medals or something like that when you collect polished building and something like "at level 80 you get 1 extra/random reward for 1 polished building collected multiplied by 15.
I would prefer something like seed vault or something like "by opening the GBuilding, you can see a 3rd bard. at level 80 select 10 or 15 buildings in your city from the list. the GBuilding auto collect those buildins in the selected order".
FoE Team, if you put a bonus like auto aid in this GBuilding, you will show us that you just want to troll us since SAM age . . .