I doubt Johnny and Sal will ever return unless in some background apperances (maybe), but what Disney's trying to do nowadays like what they did in TM'11 is that they're trying to re-introduce the classic muppets from the muppet show. They veer away from using JHH and MT characters, because those shows didn't last long and faced problems, there are only a few exceptions which would be Pepe the King Prawn, Bobo the Bear, and Big Mean Carl all from MT. The reason for keeping Pepe around is, because he's so popular fans would rage if Disney kicked him out though they tried to keep him out of the spotlight in TM '11 giving him only a brief speaking cameo scene. Bobo is also basically just too popular, though he was in a supporting role in TM'11 he served as an antagonist, so he's not taking the spotlight. Big Mean Carl is just a background character, they used him in LGatMHS, and well he's just filler like some other MT characters who appear in the background nowadays. Johnny and Sal were kept out, because they aren't that popular outside the muppet fandom, and think about it, the projects they were in weren't very appreciated unfortunately like MT, MFS, IAVMMCM, and MWoO, so they overlook the comic duo in favor of giving the films and shows negative reviews, and if you were to ask moderate muppet fans about them they would'nt know. The downside of this all is we see little of
,
, and the rest, but the upside is awesome. Characters who were neglected in the 90's come back to life like
,
,
,
,
,
,
,
,
,
,
,
,
,
, and so many of our other favorites. In the 90's when Jim and Richard left so did Dr. Teeth and Janice, and without them the writers said, "Why would we keep Floyd, Zoot, and Animal prominent when two of their beloved band members are inactive, so we'll give them lines now and then, but mostly keep them in the background". Rowlf and Scooter just were gone for the sake of Jim and Richard. Fozzie and Piggy were O.O.C throughout the 90's, because of Oz's directing career and were less prominent. The writers also phased out all of Jerry Nelson's characters like
,
,
,
, and others and only giving him one recurring role from 1992-2003 which was
, but by the time they realized their mistake of phasing out the characters of a healthy muppet performer it was IAVMMCM, and poor Jerry began to become ill, and IAVMMCM was basically the first time since when Henson was alive that Jerry Nelson got to perform his classic muppet show characters in prominent roles, unfortunately it would be one of Jerry's last muppet performances. The few handful of TMS characters who stayed normal in the 90's were
,
,
,
,
,
, and
. So with this the writers had to write in all these new characters to fill in the gaps of the prominent ones. By IAVMMCM the origanal TMS cast was slowly returning to prominence and by LTS Disney decided to turn the table and end the reign of muppets like Johnny, Sal, Clifford, ect. and retire them. And that's the story of why Johnny and Sal have little chance of returning to prominence, unfortunately.