You’re suggesting 2 down and 2 up, with a further playoff between a D1 and D2 club. That makes 3 up, 3 down, and 12 teams in D1 next year again.
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Btw when we are aiming for 10 teams in D1, there is not a whole lot of point in finding ways to get the teams in 3rd or 4th in a 12-team D2 an opportunity/playoff to get into D1. Speaking from experience throughout the noughties, when you’re the 15th-16th best team in the county, D1 isn’t your level.