Last night the British Embassy held an information evening on UK citizens' rights in Germany post Brexit.
The event was filmed and will be posted on their website; it's worth watching because it gives strong indications of how Germany would plan to deal with us, in the case of Deal_Brexit and No_Deal_Brexit. The German speakers were pretty reassuring tbh.
However the Berlin Ausländerbehörde stated that they already have plans to try and process the 18,000 British in Berlin: in the first week of January they will post a link on their website, in both English and German.
We are to register there and (IIRC) we can download and print a document which confirms our right to reside and move freely in Germany (and presumably by default Schengen area?).
The document ~might~ help at passport control coming back from the UK but it's not a recognized legal document in that scenario.
But it's best if you check this out for yourselves as this is only my understanding of things. (and it was after work and there was no food and I was tired :D)