I'd prefer two conferences of eight. Home and away with top 2 from each going into semis and then final. For the finalists that's a 16 game max league season.
16 teams would cover all those with really serious professional pretensions. Three year lead time with the 16 chosen on properly examined financial and infrastructure criteria, ignoring league positions at particular points in time.
If we're seriously looking at ring fencing then in an ideal world I'd want some kind of draft system to ensure that all the clubs get a similar flow of talent. In my ideal world I would also want a large proportion of playing rosters and match day squads to be English qualified (there's a world surplus of lawyers, surely one of them's bright enough to find a way to achieve this).
Would also like a good geographical spread, but wouldn't run the conferences on geographical lines as that would all get a bit samey and I wouldn't want Harlequins fans to miss out on trips to Newcastle and other Northern laybys. The US is so vast that regional conferences make sense, but England's really not that big. My suggestion would be that the two finalists from the previous season would be in separate conferences but beyond that, to keep it fresh, the conferences would be drawn at random each season.
Financially, any funds from the RFU, TV, investors etc to be distributed evenly with no concept of prize money. In addition a cap would apply to the amount of external sponsorship each club can obtain.
Simples!