A "planar handbook" by a 3PP? There is one, "The Path of the Planebreaker" by Monte Cooks Games. This is one of the titles should appear as licence in D&DBeyond.
My vision about the 5e Ravenlof is Vecna wanted, and he wants, vengueance against the Dark Powers, and he used Azalin Rex and his wishes to escape as a puppet to cause the wrost possible troubles. The previous dark lord of Falkov was killed (a tyrant like them earns too many enemies). Maybe a rebellion by the soldiers because these didn't want an useless sacrifice, or because they feared being punished because they couldn't obey certain orders. The previous "Core" could still exist, but this time without dark lords (the demilords are a different thing). Some dark lords were terminated, and then the dark lords got "variants" from other places of the multiverse. The new Ravenloft has been redesigned to allow space for future new elements.
I see 3.5 eladrin now they are archon and all no-evil outsider is celestial monster type.
Bariaurs as PC species can't be only centaurs with horns... (and now I rebember Doric, the tielfling druid in the action-live move was wearing an armor for moments in the sequence of "wild shape"). And creatures with four legs and hooves can't climb in the same way than ordinary bipedal humanoids. This may be relevant if characters wanted to climb a tree, to use a climbing rope or a ladder. Maybe the goat-like hooves by bariaurs could work as opposible thumbs and they bariaurs can climb better, or faster, than centaurs. Can centaurs to take on and take off underwear without help? Should they wear horse sheets to cover "those zones"?
My theory is the planar handbook will be later the new setting after Planescape, and this will be a spiritual succesor of Dark Sun, the same crunch, and adding the 3.5 Expanded Psionic Handbook+The Complete Psionic but without the Athasian Tablelands, but a different zone, where the slavery is possible, but not mentioned.