Ask online and most people will tell you that the right system to run Planescape with is AD&D (2e) - the system the game was written around.
Assumption no.1: D&D (2e) is the core Planescape system.
Look at Planescape: Torment, the game that made most people fall in love with Planescape (who weren't already fans) and you'll find out that it takes some great liberties with the AD&D system to achieve its atmosphere. Look at the original Planescape Boxed Campaign Set and you'll discover it's mostly fluff. There's almost no mechanical content in it, despite the fact that it follows some pretty different roads to the generic implied D&D setting. There isn't really anything in the source material that speaks to mechanics.
Assumption no.2: The core system can be tweaked significantly without effect to the setting (and vice versa).
Assumption no.2.1: Extensive hacking and houseruling can be performed to achieve the desired game effect.
A few of the important thematic elements of Planescape are: Alignment, Factions, the power of Belief & the Rule of Threes.
Assumption no.3: These elements can be implemented into the mechanics to strengthen the flavor.
Six ability scores. Combat & noncombat proficiences AKA "skills". An "Armour Class" score (that covers more than just armour). Saves/Defenses. Race. Class. Hit Points. I think these are the operative words of a "baseline D&D". The way they work has largely changed from edition to edition, but there are common threads.
Assumption no.4: A Planescape system should retain those elements in some capacity at its core.
Assumption no.4.1: Since we will be adding to the core, keeping it minimal is desirable. (Also because elegance and minimalism is a design ideal.)
Next up: the rules. (not immediately)