I'm still uncertain about Planescape. One reason WotC cancelled it back in the 90s was because they felt planar adventures should be part of core and not spun off into their own setting. During 3e, there was some amount of core material on the planes that built on Planescape. Even if they do go with Planescape, it's not likely to be presented as a separate setting but something connected directly to core. That shouldn't be a problem even with the Realms being the default now, since Toril was one of the more important worlds on the Material Plane in Planescape.
Though it's also possible a Planescape product could talk about ties to various Magic settings, given how planewalking is a thing in M:tG. That would make me laugh too, as TSR in its last days tried suing WotC over the terminology of planewalking. A M:tG/Planescape crossover would be a hilarious distant finale to that.