I had a model that I threw together a year ago, which I'll summarise here. (
edit: the table formatting screwed up and I can't really be bothered to fix it at the moment.)
Tier One is created from the top 10 teams in the world rankings. They play a home-away round-robin of eighteen games over three years, for a total of six games per year.
There are nine other weekends per year designated for international rugby, which allows for the Rugby Championship, Six Nations, and mid-year tests to remain as desired; this also avoids the problem of teams outside Tier One being prevented from developing through exposure to higher-level teams.
In the third year, two of the nine "other" weekends are given to a set of finals: 1st hosts 4th; 2nd hosts 3rd; winners progress to a final, hosted by the upcoming World Cup host nation. The winner is the Tier One champion.
Tier Two is created from the next 8 teams in the world rankings. They also play a longer set of games, with a triple round-robin of 21 games over the three years, or seven weeks per year; this leaves them eight "other" weekends per year: enough for their current annual competitions, but not enough for a full three-week mid-year series.
In the third year,
two of the eight "other" weekends are given to a set of finals: 1st hosts 4th; 2nd hosts 3rd; winners progress to a final, hosted by the upcoming World Cup host nation. The winner is the Tier Two champion.
Tier Three is created from the next six teams. They play a quadruple round-robin of twenty games over the three years; seven games in year one and two, six games in year three. The team on the top of the table after twenty games is the Tier Three champion.
Becoming Tier One champion is designed to be second only to the winning the Rugby World Cup.
Becoming Tier Two champion is the only way to enter Tier One, barring World Rugby disqualification, etc. The Tier Two champion replaces the 10th placed team in Tier One for the next World Cup cycle.
Becoming Tier Three champion entitles the team to play the 8th palced team in Tier Two. The winner plays in Tier Two for the next World Cup cycle, the loser plays in Tier Three.
The 6th placed side in Tier Three is always relegated. The highest-ranked team that is not currently in the Tiers model takes their place in the next World Cup cycle.
We made this model using the rankings from immediately after the 2011 RWC, so imagine the initial setup as:
Tier 1 Ranking Tier 2 Ranking Tier 3 Ranking
New Zealand 1 Samoa 11 Namibia 19
Australia 2 Italy 12 Russia 20
France 3 Canada 13 Uruguay 21
South Africa 4 Georgia 14 Spain 22
England 5 Japan 15 Chile 23
Ireland 6 Fiji 16 Portugal 24
Argentina 7 USA 17
Wales 8 Romania 18
Tonga 9
Scotland 10
If we imagine the rankings at the end of 2014 as indicitive of how the finals would go, etc., then the three Tier Champions would be:
Tier One: New Zealand
Tier Two: Samoa
Tier Three: Russia
Highest non-tiered team: Germany
The associated losers would be:
Tier One: Tonga
Tier Two: Canada
Tier Three: Chile
As a result, the following cycle would be set up as follows:
Tier 1 Ranking Tier 2 Ranking Tier 3 Ranking
New Zealand 1 Japan 11 Canada 18
South Africa 2 Fiji 12 Uruguay 20
Ireland 3 Tonga 13 Spain 21
England 4 Italy 14 Portugal 22
Australia 5 Georgia 15 Namibia 23
Wales 6 USA 16 Germany 24
France 7 Romania 17
Scotland 8 Russia 19
Argentina 9
Samoa 10
Run through again, and, if we use the same assumptions about the end-of-2018 rankings as indicitive, then the
three Tier Champions would be:
Tier One: New Zealand
Tier Two: Fiji
Tier Three: Uruguay
Highest non-tiered team: Netherlands
The associated losers would be:
Tier One: Samoa
Tier Two: Russia
Tier Three: Portugal
And that would make the competition in the next World Cup cycle look like this:
Tier 1 Ranking Tier 2 Ranking Tier 3 Ranking
New Zealand 1 Japan 11 Russia 19
Ireland 2 USA 12 Canada 20
Wales 3 Georgia 13 Spain 21
England 4 Tonga 14 Namibia 22
South Africa 5 Italy 15 Netherlands 23
Australia 6 Samoa 16 Germany 26
Scotland 7 Uruguay 17
Fiji 8 Romania 18
France 9
Argentina 10
Anyway, the point is that it gets every top team to play each other, and it recognises that the situation in international rankings is pretty fluid (apart from #1, sadly). The spare weeks should probably be committed to playing teams in the other tiers, which would help reduce the gap further, but we weren't able to come up with a reasonable model for this.