Gonzo should be on the season 4 box and Scooter should be on the season 5 box. Of all the characters on The Muppet Show, I think Scooter would be considered the fifth main character (during season one, Scooter would have been fourth, and Gonzo fifth).
And if they release the sets that way, that takes care of the top row of Muppet Central smilies.
It'll likely be that the fifth box will have one of the smaller, but outrageously popular characters on it like Beaker or the Chef or Animal. Beaker would be a very cool cover, but I really hope it's not the Chef or Animal. Am I the only one who finds Animal's popularity as annoying as Elmo's?

Not that I dislike Animal at all, he's hillarious when he's not placed in a lead role, as in Muppets From Space. It would be a poor choice to have him on one of the next two box covers.