It would be nice to see them break up the Monster Manuals by CR ranges. A low-CR book, a mid-CR book, and a high-CR book would be great. That way, people can just get what they need, when they need it.
As someone who DMs and plays in high-level campaigns, I've gotten quite good at advancing monsters. But sometimes its nice to have a creature that is designed to be high-level. It mainly comes down to the special abilities: high-level creatures often can do things that low-level critters reasonably can't...and the rules don't provide for adding these abilities when advancing creatures. Sure, you can use templates, but that's extra work and sometimes has extra baggage that doesn't fit the creature's concept.
That's why I like demons and dragons so much...you've got one at nearly every CR, so everyone wins.