One thing's for sure: If somebody like Nelson, Hunt, or Oz got a documentary, it would be disapointing if it were to focus almost exclusively on one particular character. With Clash and Spinney, it makes a bit of sense.... Elmo has taken up most of Clash's time in the last decade, though he has performed plenty of other characters as well (though it seems most of Clash's other characters sort of disapeared around season 31, then Hoots, Natasha, and maybe Benny came back around season 35, and then it seems they're all gone again, though I haven't watched much of the last season), and of course Spinney hasn't had many characters besides Big Bird and Oscar.
Though I wouldn't be surprised if a Dave Goelz documentary/autobiography had a title relating to Gonzo, or if one on Steve Whitmire had a title along the lines of taking over Kermit.
Hmmm, and while Jim Henson has had many documentaries, he hasn't really had one in a long time. It would be kind of great if Henson got the documentary film treatment. I think there is a lot of stuff to cover that wasn't acknowledged in The World of Jim Henson.