So apparently there's a fifth model & it's the one WR (World Rugby) & CVC (who now own 1/7th of the 6N) prefer:
* biennial (presumably in even-numbered years when there's no RWC or Lions)
* 6N top-four in one Group, RC top-four in another (implying RC would be expanded)
* second tier Europe & Pacific Groups with (automatic?) promotion/ relegation
* SH/ Pacific Groups host 2 x 6N/ Europe teams in July, visit the other 2 in November
* playoffs also in November (so always in NH one imagines) over three weeks (implying QF, SF & GF/ playoffs for 3rd & possibly 5th, shit why not 7th & 8th as well)
* November window extended by a week, 6N window loses a week, July window stays the same
So on current standings etc it's;
6N Group: Wales, France, Ireland, Scotland
RC Group: NZ, SA, Australia, Argentina
Europe Group: England, Italy, Georgia, Spain
Pacific Group: Japan, Fiji, USA, Samoa
A revived Nations Championship in 2024 could see the All Blacks meet the top side from the Northern Hemisphere in the final.
www.stuff.co.nz
I'd hope there'll be at least a third & ideally a fourth tier so the likes of Germany & Namibia have something to sim for. You probably wouldn't go straight to automatic promotion/ relegation between tiers two & three but that should be the long-term goal imo.