One thing that I wish I would see more of in the canonical descriptions of the planes is how exposure to a given plane transforms a prime/planar. We get touches of it on Pandemonium (prolongued exposure to the place give people psychological twitches) and the Beastlands (that whole turning partway into an animal thing).
It's usually depicted as something that only happens while a person is on the plane in question, but being on a plane should really be a transformative experience for people. I think it could be interesting to explore how the planes chane people and what sorts of planar tics that planewalkers develop.
I need to get going, but I wanted to capture this thought before it slipped away.