Rebuilding FRA is like that old saying:

"If you want to go there I wouldn't start from here."

They do get a one-off chance with terminal 3. If they brought that into service early, move all non-*A airlines from to 2 to 3, renovate 2 so it's ready for better integration with 1, then use 2 for *A while reworking C's integration with A/B, bring C back in while renovating B and finally use everything else while making changes to A/Z.