As much as I love the setting a lot of it breaks down in play, especially the "philosophical" aspects. The mechanics of "belief is reality" as mentioned, for example. The factions are another example--their philosophies are caricatures, don't make sense, and most importantly don't inherently lead to game-able, adventuresome situations.
Second, the planes are many and they are infinite, and this makes a setting book inherently difficult. The planes and variety of planes don't make sense...if it's based on the alignment system why not just have 9? Large portions of the upper planes are just infinite pastoral landscapes where everyone gets along. Each plane has some idiosyncratic number of layers that don't really add much variety to the plane itself. 2e's solution was to spread the material over 4+ box sets (plus guides for the astral, ethereal, and inner), though some of this can be attributed to TSR's "business" practices of that era. But what was worse is that while all the box sets have interesting ideas, they are a) buried in mountains of prose, b) often not game-able (where is the conflict in bytopia? How does anyone survive the abyss? etc), and c) still only one little piece of infinity. Further, the designers often went too far in offering typical dnd-fare, so that for some reason you knew what kind of armor is available for sale in carceri or the name of the halfling tavern owner in arborea.
Finally, planescape is not planescape without DiTerlizzi's art. They might not need his art in particular for a new setting (though I'm assuming they still own it), but they do need someone with a unique vision to make it new and weird. If they do planescape in 5e's fantasy superhero art style (with 5e tieflings) I'm going to be sad.
That being said, here's what they could do: a Ravnica-style book covering Sigil, with new factions, and with modular adventures to go on both in the other planes and within sigil (especially undersigil). The book should be full of steampunk whimsy mixed with dark city aesthetics. The planar adventures would cover specific locations and provide examples how weird, dangerous, challenging environments for characters of all levels.
That being said, one thing I hope they do not do is actually cater to us old fans. I was disappointed to the reaction to the ravenloft book in some quarters, with fans saying it had betrayed the original in x, y, z ways, and what they wanted was the same setting but with an updated timeline. That's very backward looking, and especially silly given a setting like PS which deals in terms of infinities, both spatial and temporal. The ethos of PS is forward thinking and genre breaking, and I'd rather the designers and writers experiment and risk producing something not to people's tastes for the aim of producing something truly new.
That being said, I'm not sure anyone at wotc is up to that task, so I guess I'll have to go play Electric Bastionland.