I don't know if anyone on here follows rugby league, but they have done something, similar to something suggested above, that works well.
Ignoring the amateur scene that is obviously impossible in speedway, a few years ago they had just 2 pro division the Super League (read the BEL) and the Premiership (or NFP) (read the BPL). The SL had 12 teams with the NFP having 18/19. In theory there was 1-up-1-down promotion and relegation but it never happened because of the gap. There was also week in week out thrashings in the NFP due to having too big a variety of teams and new teams were put off joining, there was also nothing for the bottom teams to play for. Eventually the NFP was split into National League divisions 1 and 2 with one amateur team (in London not a RL area) also deciding to join. They also created a NL3 (like the BCL) with no promotion and relegation, but to bring teams up to a level where they can join NL2, and my club Coventry Bears (again not a RL area) won NL3 this year and are applying to join NL2 for 2006.
P&R works where the top 6 playoff winners of NL1 can apply to join the Super League subject to their business plan and ground being passed as suitable, if it passes they switch with bottom of Super League. The top of NL2 automatically replace the bottom of NL1 and the 2nd bottom of NL1 enters playoffs with 2nd to 6th in NL2 to defend their place. There is a tiered salary cap between all the divisions. There's still a pre-season cup with NL1 and NL2 starting with regional groups before the knockout stage.
What does this mean for speedway? Basically if you split the BPL into 2 divisions, not only will the top division be closer to the BEL, so P&R subject to minimum criteria would work, but also it would be less of jump for BCL teams wanting to join the BPL division 2. Have a pre-season cup to give some variety to the small divisions of the BPL and keep local rivalries, e.g. if Edinburgh and Glasgow end up in different divisions. This would be the Premier Trophy. The playoffs would give more teams something to play for, at the moment any team who can't win the league has nothing to play for in the BPL