The UK Government has announced its new policy on EU nationals coming to the UK after Brexit : https://www.gov.uk/government/publications/eu-immigration-after-free-movement-ends-if-theres-no-deal/immigration-from-30-march-2019-if-there-is-no-deal#third-country-family-members-accompanying-eu-citizens
As regards ID cards the UK will be phasing out the use of EEA national identity cards for travel to the UK in 2020. More details will be provided in due course. The paper is to outline arrangements for a transition period between 31 October 2019 and 31 December 2020 during which EU nationals will be able to move to the UK to live, work and study by applying for a 36 month temporary immigration status - European Temporary Leave to Remain (Euro TLR). This TLR can be used as a route to applying for permanent settled status.