They cant just "join" as there will be insufficient riders to make up a team...Take Leicester's 2025 team as an example.
Fricke, Douglas & Becker, gone from the UK if one league comes along as Poland has first claim.
Howarth, Masters, Kemp & J Thompson ride for existing clubs.
A similar scenario exists at the other remaining 2025 Prem clubs
Remember doubling up disappears in "one big league" and the only source of new riders is at the bottom of the capability chart - NDL riders - some of whom cannot get anywhere near place in a current championship club.