Chief EU Brexit negotiator Michel Barnier said last week in a meeting at the European Parliament that "frictionless" EU-UK trade could continue if the UK were to be a member of the European Economic Area EEA and also a customs union with the EU.
Currently Iceland, Liechtenstein, and Norway are members of the EEA but not of the EU. To do what Barnier suggests would likely require some changes to EEA membership rules, particularly to avoid a tourism-killing hard Ireland-NI border.
Anti-EU Brexiteers in the UK government are sure to resist this proposal.
Stay tuned.