I believe the existing rules are very good. Regions rank all candidates from their region (including MPs), and then a National List Ranking Committee (with a majority elected by electorate delegates) combines them into a national list, making modifications as it sees fit.
I’m not a fan of any moves to exempt sitting MPs from the normal process or to guarantee in advance all MPs will be in safe list places. I think it weakens the party to have less discretion over how the list is ranked. There is of course always a natural tension between the organisation and the caucus on these issues, but I have always felt that one of National’s real strengths over Labour has been a far more democratic candidate selection process, and not protecting incumbent MPs in list ranking, as Labour did in the last three elections. Part of why Labour is now facing problems is it left its rejuventation too late.
National has a proud history of trusting the collective wisdom of its members. If National had rules protecting incumbent MPs, then Brian Neeson instead of John Key would still be the MP for Helensville. Likewise Judith Collins and John Carter woud not be MPs.
MMP especially poses challenges for parties – not just in list ranking. You also have situations where List MPs and new candidates compete for new seats after boundaries are redrawn. While Selwyn has eventually resulted in a very good outcome, I think there are lessons to be learnt there about not trying to scare people off from standing against an MP – it tends to be counter-productive.
Now having said all that, I don’t think any National MP will be or should be retiring against their will this election. Of the 48 MPs, five are retiring (Connell, Rich, Blumsky, Clarkson and Simich) so that will leave 43 incumbents. On current polls if National should get at least 55 MPs, so there should be at least 12 new MPs joining the Caucus. And there have been some excellent new candidates selected.