There is a simple solution, make the AB slots available to any interested airline. FR would be flying within 4 week and bring cheap tickets to the German market.
Ryanair had the chance to fill these gaps, and the still have its but for many routes like the domestic shuttles they aren't interested. Even now it seems the have problems out of FRA, as their fares are often considerably higher than from other airports.
If no other airline is managed well enough to make domestic german work, or no other EU airline is willing to step in, then let it be so.
Most people on the domestic routes are travelling for business anyway and need flights to their schedules, and I doubt FR would be willing to fly hourly 3/4 empty between FRA, MUC, HAM, TXL, DUS. It just isn't their business model.