I've said a bunch of times, there's something inorganic about Elmo and Abby. They're the most popular characters and all, but they seem so mechanic together. Elmo works wonders with Big Bird, Oscar, Zoe, Grover, and a couple others... Abby works great with some of the other characters... but when they're together, it's like Abby can't screw up her magic and Elmo can't emote. So basically, they just don't play off each other as well as they want them to.
Let's say Disney made a project with Mickey Mouse and Pooh together, as they're the most popular and money making characters they have. They just can't play off each for various reasons, but it sells merchandise and keeps kids tuning in. I'd rather Mickey go with Goofy and Donald and Pooh with Tigger and Rabbit... characters they play off of that have some measurable tension or some sort of unique chemistry with. That's basically the best example I could think of...
I mean, Abby and Elmo sort of work, but they seem more like they're together due to demographics. And really, Elmo has couple chemistry with Zoe... If I were a 13 year old girl, I'd draw humanized anime teenager Elmo and Zoe couple pieces on deviant art... that's how much like a couple they are. Abby seems like a friend of a friend of Elmo's and they just wind up being in the same place together.