In the UK, Open leagues are how almost all of our team sports work, professional or amateur. It all hinges on a divisional structure, with promotion or relegation.
The competition that includes something closest to your concept of nationals is our National Club League, or NCL. That works like this:
The National tier contains four divisions of eight teams. Those are determined geographically - Midlands, North, South East and South West. Each team will face all of the others in the same division; at the end of the season, the top team from each division will progress directly to the finals (a knockout competition between 8 teams), whilst a further 8 teams enter the playoffs, from which 4 will progress to the finals. To give an appreciation of the level at the top, it truly is open, and contains world ranked players. Liam Broady played for the champions last year. The bottom two teams from each division will be relegated to the Regional tier.
The Regional tier contains eight divisions of up to eight teams. These are again determined geographically, but are more concentrated than at National tier. Each area has two divisions - the Midlands is split into East and West, the South East is split into North and South for example. The winner of each division is promoted to National tier, the bottom two teams in each division are relegated to the County tier.
County tier is the smallest geographically. Division 1 will typically contain up to 8 teams, and the winner will progress to playoffs where they play for promotion to the Regional tier. The bottom teams are relegated and replaced by the winners of Division 2, and you rinse and repeat below that with Division 3, 4, 5… however many you need to accommodate all the teams. Division 1 will typically feature players with UTRs of up to 10 or 11. You occasionally get someone better, but for the most part players of 12 or up will either be playing in a higher division, or not playing league at all and focusing on tournaments.
As well as NCL, we have plenty of leagues within each county. Summer league, winter league, mixed, seniors... They all have a similar structure, with promotion and relegation. There are no play offs in these - if you’re the best team in the county you’ll get a mention in the newsletter, but that’s about as far as it goes.
The promotion and relegation structure gives almost all matches a measure of importance. If your team is performing well you’re aiming for promotion, if you do badly you’re battling relegation. It means you don’t get many dead rubbers, which is good for competition.