Why not introduce a McMahon system tournament for 8 teams (as suggested elsewhere on RRF) Then the four strongest Nations play each other, and the four weakest Nations play each other. 1-4 from each pool play their opposite number.
I'd not previously come across McMahon, but am familiar with Swiss tournaments. I can't see any kind of Swiss working on a home and away basis, because of travel/accommodation bookings and such. There are no pools in Swiss, nor in McMahon as far as I can see. The only difference is on initial ranking for the bottom teams, which is somewhat ok for #6 of 6N, but potentially harsh for the #5, particularly in terms of gate revenue. That said, a Swiss system only takes three rounds to get "a" winner when there are eight teams competing.
No country wants to lose out on matches against the others, particularly home matches for Home Nations and France. However, team sides losing players don't want even more matches.
So pools it has to be, somehow. How? A double four nations, isles and continent, using the middle weekends for matches that don't count towards competition.
Weekends 1 & 2 for the home nations, would be against two of the other home nations, on a rotating basis, one home and one away, in a six-year cycle. France and Italy would host the top other continental countries one year for these first two weekends, and play each other on the second weekend, alternating home and away in a four year cycle.
In week 3 - currently the first rest weekend - the home nations would play their final home nations match on a 'friendly' basis, but still very much for ancient trophies, and perhaps as a tie-breaker if bonus points don't suffice.
The schedule for weeks 4-7 would depend on results from the
previous years, on a 12-year cycle, with weekend 5 being a special case in which the French and Italians would compete with the two lowest-ranked home nations on a friendly
Obviously this isn't a complete proposal, but the essence of it is to split the competition in to four Swiss-style pools that each have four teams and hence only two rounds, which are played in weeks 1&2 of the previous year and weeks 6&7 of the current year. Week 4 remains preserved so that the current six nations are likely to play each other competitively. The greatest weakness of the system is that France probably only plays three of the Home Nations, but it - like Italy - is guarantueed two home fixtures which in the case of France could be used to give promising young internationals a place in the starting side. Both are guarantueed their place in the top 8 for all time, but neither can pick a B team in the opening weeks either.
It would make sense to have the final two weeks end in #1v#2 and then #1v#1 of the different pools, which makes week 4 be #1v#3 and #2v#4 cross-pool matches. Calling them H and C for home and continent, that makes week 4 be 1Hv3C, 2Hv4C, 3Hv1C and 4Hv2C; week 6 becomes 1Hv2C, 2Hv1C, 3Hv4C and 4Hv3C, and finally week 7 be 1Hv1C etc. Of course this makes being #1H likely to play France last....
Although this isn't a fully serious proposal either, it could be run for a transitional period of 12 or even 6 years before transitioning to a Europe-wide system of 4-country groups with relegation and promotion on a play-off basis, and either a home-and-away structure of 6 matches or traditional matches such as the Culcutta Cup being played as pre-tournament friendlies.