A group of British expats in the Netherlands, 'the Brexpats', have brought a case arguing that EU citizenship cannot be revoked and therefore all British nationals will remain EU citizens post-Brexit. The case has been referred to the ECJ.  

It seems unlikely that their arguments will succeed, but the case is worthy of attention, because if the ECJ were to agree they are correct, then this would have serious implications for Brexit negotiations, as it would mean British nationals retain free movement rights, even if other EU nationals lose their right to come to the UK.