It would be absolutely great if Disney pulled some strings and let in a cameo by one of the Sesame Street characters, though that's impossible. Disappointingly impossible, but we have to move on.
Anyway, I have to say this... we gotta stop saying "we want X obscure character in this movie." Two reasons... we'd be disappointed if they didn't appear, and if they did, we wouldn't be surprised.
And one more bonus reason. I think that if they focus on the characters that always had larger roles in the original movies, we'll get stronger writing from those characters, leaving the background characters to be there as support. That is, the characters who were always in the major focus in these films- not just Kermit and Piggy... but Scooter, Rowlf, The Electric Mayhem, Bunsen and Beaker, Sweetums... Sure, I really want to see Link have a major speak movie role at some point, but I just wanna see Rowlf and Scooter used in more than one scene where they say one short line for fan service.
As for Pepe, again, I'm sure that the supervising heads of Muppet Studios will vouch for him, and they'll work him into the story somehow. They clearly intend to merchandise the character at some point... he's on tote bags and towels and a Kid's T-Shirt... no adult one yet, grrr...