Speaking of MoP, am I the only one sort of irritated by how hard it now is to move between alternate worlds in the Prime Material, with the strong implication that the entire Great Wheel is centered around a single Prime Material world? (scoff, scoff)
While things eventually got VERY silly in 2E, I like that there was a way of doing the really old school way of planar travel, from fiction: Moving between Earth and fantasy realms. It'd be hard to model, say, the Chronicles of Narnia in default 3.5 D&D now, whereas there would be any number of ways to move between Earth and Narnia in 2E, which is true to the novels.
I'm thinking of just grafting Earth onto the Prime Material as another plane like the Plane of Shadow that only links to a few Inner Planes. Fiends, or whatever, that make it to Earth, have to cut through another Inner Plane to get there.
(Of course, I also toss in Faerie, Time, the Far Realm and the Mirror Realm, just because they're so freaking cool. And if I ever have an Asian-themed area in my campaigns, you better believe that you'll be able to reach the Elemental Plane of Wood from there.)