Moderator note: Split from https://www.railforums.co.uk/threads/eurostar-possible-expansion.199790/
Once the Brexit transition period ends on 31/12/20, the EU will presumably insist on much more stringent control for travel between the EU and the UK, which will be a "3rd country", so all stations served by Eurostar trains will require appropriate infrastructure and manned security barriers.
This is likely to have a major impact on travel via the Channel Tunnel, and will effectively quash any significant potential expansion of Eurostar destinations that might have occurred if Brexit hadn't happened.
I expect that security and document control arrangements will need to be similar to those required for travel across the eastern frontier of the EU into Belarus and Russia, albeit presumably with some visa-waiver arrangements.
The EU will presumably also insist on strict controls for travel across the Irish Sea, including between Great Britain and the 6 counties.