I think Walt would appreciate the storytelling style of Phineas and Ferb. It's wry and sharp but remarkably kindhearted. As for the way the show is designed, hmmm...a little too abstract and minimalist, perhaps, for Walt's tastes. You have to admit that the Phineas and Ferb walkaround costumes are a bit...startling. They don't exactly fit into the rest of the walkaround gang, visually speaking.
There's been a lot of griping about Hannah and the various Disney Channel tweencoms, but after all, in Walt's time, there were shows starring teen Disney celebs like Annette Funicello. I don't think the teen-focused programming on the Disney Channel is as much of a departure from tradition as some have suggested. Although I don't like the shows at all, they're not aimed at my demographic, so that's not surprising. However, I do appreciate that Disney is providing tweens and teens with programming that's relatively wholesome, and I think Walt would approve of that.
I don't think Walt would have purchased the Muppets. Period. Or Marvel, for that matter. He preferred to buy a property or concept and then remake it into HIS version of that property. That's not being done with the Muppets or Marvel, as far as we know. Therefore, there is likely to be none of the traditional Disney artistry involved in their acquisition, and anyway, what does Disney need with either of them?