So there is agreement "in principle" that UK citizens (subjects, actually, but still...) will keep the right to stay after Brexit:
https://www.gov.uk/guidance/advice-for-british-nationals-travelling-and-living-in-europe
No mention from the UK of any contingency plan if we crash out with no deal at all.
What should we expect? Try and get a visa if we haven't got our German citizenship by then? What kind of visa would be appropriate?