I thought of an great idea for a mini tournament.
Generally speaking the top three SH sides always tour the NH in the autumn (I assume this year will be an exception on account of the RWC), so why not make it a proper tournament: the top two ranked NH sides against the top two ranked SH sides in a league plus maybe a final between the top two placed teams. That's three matches for all teams plus another one for the top two, which is the same number, roughly as they play now. The tournament could be alternated with the South, but the NH sides' home grounds are conveniently close to each other.
I've arrived at almost the same idea as PD but mine is smaller scale and can accommodate the Six Nations every year. In fact, if the qualifying NH sides are the winners and runners up in the previous 6N, it adds something to that tournament.
I'd really like to see some sort of tournament at club level too, but I can't see that being financially viable.