My structural suggestion hasn't changed too much in 20-odd years since coming up with it.
Of course, it requires coins to see each other as collaborators, not antagonists.
Europe: (NB, Africa is a different continent to Europe)
3 tournies, each of 4 pools of 4
4 Prem
5 URC
6 T14
+ Winner of the next level down.
3rd tier is the remaining Prem, URC & T14 + the likes of Georgia, Romania, Spain and Portugal.
I'd go with QF, SF & Final, but each tournie has a Cup and a Plate phase (no dropping down).
I'd play all pool matches in 1 block, December and January
England:
Prem of 10 teams, Champ of 14, reduced MSCs, both covered in 1 TV deal that includes 1 match from each on terrestrial each week.
Equally, both covered by the same title sponsor.
Automatic promotion & Relegation.
No SF for the Prem. Top 2 face off in the final, league points difference translates to scoreboard difference ahead of kick off.
Domestic cup involving all 24 teams - probably 4 pools of 6.
Prem club squads limited to players with fewer than say 20 (absolutely up for debate) starts (Prem, Euro & Test) in the previous season. Alternatively 18 of the 23 must be academy players/graduates for that club (again, lines in the sand are up for debate).
Domestic cup pool stages to be played during the 2 international windows (5 matches per window) then straight into SF and F (again with cup, plate, bowl, shield)
RFU get 1 training week ahead of AI and 1 ahead of the 6N, clubs play first round of that block of Dom Cup in that week.
I make this a 40 week season (1st Sat of Sept to 1st Sat of June).
Assuming AIs and 6N times are cast in stone:
8 Prem
5 Dom Cup & RFU time
2 Prem (dammit)
6 Euro
5 Dom Cup & RFU
2 Dom Cup KOs & RFU time
8 Prem
4 KOs for Prem & Euro
Presumably URC and T14 are happy enough playing through international windows, when we'd be playing Dom Cup.
I'd be okay with the old HC solution of filtering the QF at least into that post-6N block of club rugby.
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Theoretically, T14 fits perfectly into the same structure, playing straight through the international windows; including holding their final the week before the Euro final. Of course, they want the T14 final to be the climax of the season, but they've easy options of delaying the start of the season, or adding rest weeks in there, or both.
URC have 18 rounds, QF, SF & F - so an extra 2 weeks than us, which can be played when our boys are training with RFU, and we're 1st and 6th rounds of the Dom Cup. If they want to, of course.
IF (big if) we could then find 23 weekends (and enough players) in there from the 28 Prem & Euro weekends for a 23 team U21s (play everyone once) as well, that would be spiffing.