Too many German airports?
The problem here is that there are just too many German airports (there are too many in UK too, but the problem in Germany is more acute).
So many former NATO and Warsaw Pact air bases have been reactivated as "airports" some so close to each other that it's almost laughable. Take FKB (Karlsruhe/Baden) and Lahr (Black Forest Airport). One has a reasonable scheduled service base - the other has diddly squat. Surely one has to go.
There's Moenchengladbach - down the road from Düsseldorf, Weeze - nowhere near Düsseldorf and now surely under threat from Ryanair's new policy of flying to primary rather than secondary airports, Kassel, as has been said, Zweibrücken, Altenberg (which Ryanair once called Leipzig), Memmingen serving the Allgau - or Munich if you're Ryanair.
With Germany's much superior autobahn and rail network (superior that is to UK) there's just no need for an airport on every street corner. The market, and therefore the airlines just doesn't need them all. However they are usually the sort of prestige projects much loved by local politicians so it may take a lot to get many of the closed, unless there is pressure from local tax payers.
The airport Germany really needs (BER) might be open in 4 years time - then again it might not!