Right now all they can do is cut off the money. Downsizing the organization would come under a GOP POTUS. Overdue.
Excellent Idea, Make It So!
I prefer budgeting for immediate demolition of the curent Brutalist monstrosity. No need for a replacement building — just abolish the agency.
Turn off the power to the building, for da Erf.
Now is a piss poor time to build this new building. The rich basturd Aunt Teefers in their expensive Gucci Ghillie suits will be out there burning the building down as quickly as it is being built.
The FBI should be defunded, not given lavish new headquarters.

Fire 1/3 of all employees. Then get down to the serious lopping. Move the HQ away from DC so that the remaining employees kids do not play on the same sports teams as the criminal class.
I was wondering the other day why Congress didn’t do that. I say GO FOR IT!
I would not call what they have a dump. The FBI does not need a new office building that is twice the size of the Pentagon. This is a no brainer.