I wrote this up on the T2 forum. Thoughts.
One that I could think of is playing this over two years and instead of a 1st and 2nd division run it as a single competition with two divisions but similar to the old NZ NPC format with a Cup and a Championship. Each with two 'conferences' of 6 teams. In one we have the the 12 teams involved in the 6Ns and RC (I'm adding both Japan and Fiji to that equation) and the other those ranked 12-24 on the World Ranking/having appeared at the most recent RWC. Again split into two 6 team 'conferences'. Organised however you'll have it but I'd go with the REC as one and a mix of 2 PI, 3 Americas and 1 Africa in the other.
Over the two year window each team in the conference will play each other home and away (so for the RC, REC and 6Ns that would be as is now) plus each of the teams in the other 'conference' once for a total of 16 games. Additionally, each team will also play three teams from each of the two conferences from the other division in crossover games (which would contribute the their respective points tables) for another 6 games. Bringing the total number of games to 22 over two years. With the November window of the second year acting as the Finals.
From there each 'division' then split into their own respective 6 team finals series while the bottom 6 in each play in a 'rankings round' which alongside the finals series would go toward determining both qualification and pool placement in the upcoming RWC. Which with this in place could remain a 20 event until the Nations League grows out to 28/32 teams.
Point is, this would provide the best of both in my opinion. The structure they are angling for with the commercial benefits at both levels. While ensuring that each of the nations in the Championship level would get 6 games against Cup competition. Below this we can go with the the RET, Gold Cup, ARC and a combined Oceania/Asia regional championship of 6 teams a piece. Again split into two divisions. RET and Gold Cup in one and the ARC and Oceania/Asia in the other. Home and away over two years in their regional structures and one game against each of the teams from the other conference in their division. Top 4 from each division go through to play in a mini-tournament hosted during the Finals of the Nations League with the winner of that being promoted to the Nations League Championship while the last placed Nations League team is relegated to that level.