I believe that the format of the Rugby Champions and the Rugby Challenge Cup is the way to go. The format has been used for ages, granted with a few tweaks over time, and it works. In addition all the clubs, or almost all of them, take part in an international competition. The only issue that needs to be overcome - and it is not major, IMO - is the travel distance. The qualifying criteria is also very simple.
The field is made up of the top 6 Premiership sides, the top 6 Top 14 sides and the top 7 Pro12 sides. The 20th spot is decided by a two round, home and away, play-off between the seventh placed Premiership side and the seventh placed Top14 side.
The Challenge Cup is basically made up of the remaining domestic sides (18 in total). The nineteenth and 20th spots are taken up by two teams from other European sides through playing in a European Qualifying competition, running alongside the Champions Cup and the Challenge Cup.
Both the completions are played in five pools with four teams each.
While there probably are a few cons, there are a lot of pros.
Pros.- All the players from all the clubs in Europe stand a chance of playing in an international competition.
- Promotion and relegation between the Champions Cup and the Challenge Cup do not have such far reaching effects as in Super Rugby. There are no franchises who have standing contracts with either the Champions Cup or the Challenge Cup. If you do not make it into the Champions Cup, you simply move down to the Challenge Cup and then give it your best shot in the next domestic competition.
- The main pro, IMO, is that you do not have a watered down domestic competition like the CC has become. All the players from each club is always available to play in both the European competitions and the domestic competitions. The only exception is during the Six Nations competition when the national players are withdrawn.