If Disney were to relaunch a Muppet Related preschool franchise, I'd rather they actually do something with Bear in the Big Blue House.
As for Muppet Babies, think of it this way.
MTM was the last theatrical Muppet movie made during Jim's life. The only other Muppet projects before the ill-fated Jim Henson Hour were a couple TV specials. The only major Muppet project going at the time was Muppet Babies. And like Jamie said, that kinda hurt the Muppet brand, making it sink lower and lower into the for kid's level (of course, the Play-along videos have me thinking Jim was planning to focus the Muppet brand to kids anyway, while creature shop films and projects would focus on older audiences). Muppet Babies was essentially the only major Muppet project in the 80's, and it lasted until the 90's.
Not to mention the fact that Disney has waaaaay too many brands. And in there, subbrands. Marvel can hold itself up with each character and any variation of it (though Super Hero Squad is long gone), I'm sure we'll start seeing their plans for Star Wars soon enough. Not to mention their main children's lines of Cars, Princesses, Fairies, and various preschool programming. Like I said before, Mickey and Co were finally getting recognition, but only recently has it come from something other than a preschool show mixed with nostalgic merchandising. And the Pooh Movie flopped because it was broken down into a preschool only franchise, killing the nostalgia factor and replacing it with overexposure.
I'd say a good time to bring Muppet Babies back would be after MMW, if it does well. And even so, quite a bit after that.