Was just about to post this.
They didn't do that for either of the Baldur's Gate updates. Why would they do that now?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?
Maybe, but a lot of that has just been them throwing ideas out to see what sticks. They've made it quite clear that we shouldn't see the weekly UA content as a preview of things to come. If people don't like things, they'll just get left on the cutting room floor. I'd say the only thing we can confidently say will be in the "Midway" product is the mystic.2. The UAs for the upcoming Big Crunch Book have had a lot of non Realms feeling material. Doing so also sets a framework for where and why all of these new class choices came into existence from.
I can't help but notice how close we are in proximity to April 1st...
That being said, I would love to think that it is a 5e version of Planescape...that would make me very, very happy. However, I suspect that it will likely be an announcement of the Enhanced Edition of Planescape: Torment.