Its not possible to expand the Amex any further. There is no room for 2nd tiers or upper concourses in the north or south stands, it was never designed with that in mind. Those arches are deeply embedded underground in concrete thrust blocks, they counter-balance each other to hold the entire roof in place, all the way around. The roofs cannot be raised any further to accommodate additional tiers.
What you see is what you get folks. It can't be expanded any further.
If the stadium can't go up, then shirley the only logical solution is to go down.
After Derby, once the pitch has been removed and before the new one is laid, all Barber has to do is dig down a stands worth.
Hollow out the seating and concourse then relay the new pitch.
Bish bosh job done.