tl;dr that the book(s) are easy to use at the table and lead to easy and good DM experiences, rather than being cool shelf bait.
The factions could do with a rework to be easier to use, in my opinion as someone who is terrible at philosophy. The Planescape books were great fun to read but, like many RPG supplements at the time, were designed for the shelf rather than the table, and thus didn't always do a good job of explaining how you'd translate what you'd read into an actual campaign. Whether that means tweaking/changing the Factions, or simply giving concrete and easy-to-use examples of what it actually means to be a member of one, this is the biggest thing that I'd enjoy.
Another observation that I would raise is that many Planescape books were guilty of being way too high level in their descriptions. In other words, they'd take an infinite plane, and describe lots of places in it in very minimal detail. The end result is that you'd try to describe important towns and sites in the planes, but have surprisingly little to draw on. I think that the other direction is perhaps a better idea, like the 5e modules; describe a few places in a lot of evocative detail, and allow DMs who are keen to expand do so with a blank canvas.
This comment is based off of actually using Planescape books as my high-level-adventures sources; the time that it worked well for me was when I traced a route into an Abyssal adventure site using the most interesting places detailed in the Planes of Chaos supplement (Plague-Mort, Broken Reach, Fateless Harbourage, River Styx, Twelve Trees, Hollow's Heart), since I had lots of detail and roleplaying opportunities to work with from those places. When I sent the players into Niflheim, I thought that using pre-written locations would keep things simple, but it turned out to be a lot of effort since I was basically making it all up myself.
I'd definitely be very happy with a Great Modron March storyline, since that would allow them to provide some really cool locations in each plane and cover them with adequate levels of detail, rather than try to cover everything and detail nothing. Of course, something new would be good, so long as they hit the high notes (Sigil, Blood War, Factions/Power of Belief, Weirdness, Morals are a Choice).
As always [MENTION=20323]Quickleaf[/MENTION] has the best ideas on this topic. I do agree that the Cant, though very cool to read, was in practice kind of overdone; I have however heard of a group who got so good at using it that their table was kind of incomprehensible to other tables in the gaming club, which is a cool idea.