Will it be Margret again since she didn't interview this year?
One of the regular interviewers?
Someone who won or was a hit on the show?
Or someone completely different?

It should be someone unknown to the public, like Nick and Margaret were to start with. You have to remember this person must follow the candidates around wherever they go, so they shouldn't be a distraction. And I don't think any of the previous candidates would be mature enough to step into this role, it should be someone with many years of experience in dealing with people.