6 teams from each league, challenge Cup winner and last year's winner (or next off the boat in their league). Old 4 team pool format.
Bottom seeds in the comp this year would be Munster, Sale, Clermont, Lyon and Gloucester/Racing/Sharks...
Seems good enough quality for me and a high target but achievable for Welsh/Italian/Scottish teams.
16 works too, don't see the commercial guys going for it, as does 24 but you have to give a token spot to.the Welsh and Italians that way really and the English and French won't go back on their red herring for ending the ERC.
Tigs one could work too (32 teams though dude...) but you'd have to seed it and the early rounds would be a bit meh. Maybe have quarters and semis as the two leg games.