My prediction: it will be some of all of the above - a video game update, a story arc, a supplement, a Spelljammer-esque board game, etc. If they're going to do Planescape, why not do it big? Wasn't the original plan of story arcs for them to be multimedia?
Anyhow, a true revamp of Planescape has been a long time coming. It was one of the most original, popular settings of the 90s and I was always surprised that WotC didn't do more with it in 3E and 4E.
Furthermore, this may also open up new worlds for story arcs - travel via Sigil to different worlds: Dark Sun, Eberron, etc.
Anyhow, if this truly is Planescape, maybe Dust and Midway are a Planescape story arc and setting/rulesbook. Of course that wouldn't allow for the rumored rules expansion, but maybe they're not quite ready for that and it will be in 2018.