There are still many important questions on how a primary would be carried out:
"We will shortly provide details for supporters to give their ideas on what Desmond and José should present to the seminar in Brussels. We want to start a debate amongst PES activists and supporters on how they think the selection of our candidate should be organised. How should candidates be nominated? How should they be selected? Who should get to vote in the primaries? How should those votes be counted? Should the votes be weighted like the QMV votes are weighted in the European Council?"
Personally, I think that the voting should be internal to the party, to ensure that the PES and it's policies have a greater voice in the election. A criticism of the EP from Simon Hix, is that while the EP actually represents the average EU voter quite well, representing the average voter isn't really the point of the parliament. In the same way, the PES candidate should be selected by PES members, so that party membership has value (what better way to encourage participation in the Europarties than to give membership a real meaning by opening up opportunities for participation?), and that the candidate represents a truly "PES face" in the election. As for the internal electoral system, that's a more complicated question.
Thinking aloud, the candidate should be selected on the vote of the party members (without special weight attached to MPs, Councillors, MEPs), but I think that votes should be weighted to increase the say of the member parties of smaller member states. Otherwise it could lead to the debate and participation of the smaller states being neglected. On the other hand, the weighting needs to be carefully balanced, so that the majority needed isn't so great that only bland "all things to all people" candidiates are chosen. When it comes to nominations, I think that several methods could be used: a certain number of MEPs, MPs and members as supporters could be required to nominate someone for the primaries (perhaps the "voting value" of MEPs, MPs and members could be "translatable" at the nomination stage (e.g. 1 MEP = 15 MPs)).
Finally, an interesting question is how this would impact on the border make-up of the Commission. Since a coalition is likely to almost always be needed to elect the Commission President, what form will coalition politics at the European level take? Would PES coalition partners be satisfied with commitments to certain legislative proposals in the Commission's manifesto - or will coalition partners hold out for seats around the Commission table itself? Since the EP has to approve the whole Commission, the EP has the power to force the issue if the member states remain wedded to picking their own political favourites for the Commission; while there has to be a Commissioner member from every member state, it is not a requirement that each member be from the governing party of their member state. Even at a basic level we could ask, if there's an EPP majority in the Council, but a PES-led coalition majority in the EP and PES President of the Commission, then why should the Commission be conservative by majority? The Commission has votes on proposals it puts before the Council and EP, so why should a PES (or EPP) President be outvoted by an EPP (or PES) majority in the Commission? Afterall, the Commission is accountable as a body to the EP.
It's a question for the future, since we don't even have primaries yet. But it's an important issue, so I think we should keep it in mind. As the Commission becomes more accountable to the European Parliament, and its membership a question at the European elections, the rationale for allowing its membership to almost automatically mirror the Council's will be increasingly undermined.