When Murray McCully this week predicted there would be some international organisations popping champagne corks when he steps down on May 1, he under sold himself.

The list will be a lot longer than that.

Many former diplomats will be raising their crystal gin glasses to the end of a minister who caused upheavals at the former public service museum, known as MFAT.

Many current diplomats will be quietly toasting the back of a demanding minister who wanted everything done sooner and better (word is that when unexploded Second World War bombs were found under the Munda runway in the Solomon Islands being upgraded by New Zealand, McCully volunteered half the MFAT staff with picks and shovels to find them).

Opposition parties who failed to secure his resignation over the Saudi agrihub deal will finally get the chance to wave him off.

Israeli Prime Minister Benjamin Netanyahu will be delighted to see the back of a minister who successfully got the Security Council re-engaged in a two-state solution for the Palestinians with a resolution on illegal settlements.

And National caucus colleagues inside cabinet, outside cabinet and in the backbench are excited about the opportunities that two vacancies, McCully and Education Minister Hekia Parata, will create when they leave.

Replacing him creates a dilemma for Bill English because almost every option - and there are many - comes with complications.

Many are in the frame: Defence Minister Gerry Brownlee, Attorney General Chris Finlayson, Health Minister Jonathan Coleman, Primary Industries Minister Nathan Guy, Trade Minister Todd McClay, and Land Information Minister Mark Mitchell. But more of them later.

The success McCully has had in the portfolio for almost three parliamentary terms has very likely contributed to the high interest in the post.

McCully's success was very much the result of a partnership with former Prime Minister John Key.

In his speech to diplomats in Wellington this week and in comments afterwards, McCully acknowledged Key's role and how damaging it was when there was an iota of difference between any foreign minister and prime minister.

It was a partnership that began well before they entered Government in 2008.

McCully was an unlikely candidate for shadow foreign minister, let alone foreign minister.
Winston Peters helped to set him on the path towards it.

When Labour Prime Minister Helen Clark made Peters Foreign Minister in 2005 as the price of his party's support, National needed a person with two qualities: a political street fighter to match Peters and someone who could navigate a clear policy route through the fog it had created for itself on the United States and New Zealand's anti-nuclear policy.

Foreign policy could not remain with Lockwood Smith who told visiting US Senators that New Zealand's anti-nuclear policy would be "gone by lunchtime."

It not only discredited National's position, but National's previous ambiguity had undermined Labour's attempts to repair the anti-nuclear rift because the US thought there was a back-to-normal alternative on the horizon.

McCully persuaded Don Brash to give the shadow portfolio to him.

When John Key took over the leadership a year later, with an unequivocal anti-nuclear position, it became clear that McCully and Key had already begun their successful partnership on foreign policy.

The Saudi sheep deal and the Security Council resolution would define McCully's record if column inches in news were the measure.

But one of his big achievements has been the greater importance attached to New Zealand's engagement in the Pacific and to making a tangible difference.

And the most important legacy of the Key-McCully partnership has been the repair of the diplomatic relationship with the US, to what the US and New Zealand now call the "new normal" including a resumption of US ship visits.

In parallel with that has been a steady upgrade in the defence relationship to something that may be less than allies but is more than friends.

Just what yesterday's air strikes by the US on Syria will mean for New Zealand and friends and what sort of support the United States might ask for is unclear.

But it will reinforce to English when replacing McCully that he needs someone of substance in the role.

This week he ruled himself out of taking it.

Mitchell is too junior for the role, having been made a minister only in December last year.

The main complication for the others is what they would have to relinquish and whether it would be worth it, given that Foreign Affairs might be only a six-month appointment, even if National is returned after the election in September.

Winston Peters is expected to demand the portfolio for Shane Jones, the MP-turned-diplomat who is due to join New Zealand First next month after his diplomatic contract ends and return to Parliament.

Coleman is senior enough for the job and got a taste for it when he served as Defence Minister but he would have to give up Health - for Michael Woodhouse.

In terms of proper process, Gerry Brownlee would have to give up Defence if he takes Foreign Affairs, Finlayson would have to give up the Intelligence agencies and Nathan Guy would have to give up Primary Industries.

None of those portfolios should properly be held by someone who is also the Foreign Minister because each portfolio requires important input from the Foreign Minister.

Finlayson is also too valuable in treaty settlements to be considered. Both Foreign Affairs and Trade is too big a combination for McClay at this time.

Realistically English's decision comes down to a choice between Brownlee and Coleman - and both could do it well.

Choosing Coleman, a younger generation, would be a bold signal by English that he is looking at governing for the future and is not governing for the interim, until Peters chooses the next Government.

But Brownlee has an important place in the National caucus, not just at No 4 but as someone who has forgone portfolios he wanted in order to concentrate on earthquake recovery.

He also stood aside without rancour from the deputy leadership in 2006 to allow English to take the job - although gratitude does not have to be infinite.

Brownlee has the seniority to demand it and could hand over the Defence portfolio to Mark Mitchell.

The bad news for MFAT is that Brownlee is an irascible and demanding minister.

Giving them Brownlee might be one way for them to appreciate McCully.