While Chris argues that a Planescape: Torment successor wouldn't necessarily even be the best idea for an Obsidian Kickstarter, he certainly seems to have a clear idea on how it would need to be done:
• It'd be best not to use [the Dungeons & Dragons] mechanics or the Planescape license. One reason is doing so would undermine some of the joys of the Kickstarter (not having to answer to anyone but the players – if we take a license, we have to answer to the franchise holder), I'm not sure Wizards/Hasbro/whoever knows where to take the license, and looking back on Planescape: Torment, it's been clear to me that we had to bend a lot of rules to get some of the mechanics and narrative feel we wanted. Could we have done that easier outside of a Planescape universe? Sure.
• Utilize similar writing style elements (slang, dialogue screen format similar to Planescape), depth (lots of choices per node, lots of reactivity), presentation (action descriptions interwoven in the text) and density (the Wasteland 2 backers have repeatedly asked for more text in Wasteland rather than spending resources on something else like [voiceovers], thankfully enough).
• Similar narrative mechanics. As a classic example, there's some form of morality/personality bar that's affected by your actions, although I'd want to research some other mechanic tied to the narrative.
• A camera and click-movement presentation similar to the Infinity engine isometric games. Even if the mechanics are different, at first glance, the game should share the view that Planescape did.
• Lastly, this is also something that set Torment apart – we had a good chunk of the story, dialogues and the flow of the narrative laid out before production began. This was key. If I had the power and funding to sit down for a year and script a spiritual successor out, then we built from there, I would do that, but that process is something no publisher would agree to – you're constantly under the gun, either as an internal or external developer (Josh Sawyer had to write the Icewind Dale 2 storyline over the course of a weekend, for example – he did a great job, but that's not an ideal way to write a story). Generally, you have 2-4 weeks.
A lot more interesting details are present in Kotaku's article, so I'd recommend you to read it in its entirety.
Considering Obsidian's shaky financial situation and their hit and miss track record with action-RPG mechanics, I'd be happy to actually help financing an old-school title from them with Kickstarter, and hope that chance will present itself eventually, whether it be a Planescape: Torment successor or another, maybe more daring, concept entirely.