Just want to put a sort-of transcript of Nathan Stewart's actual comments, as it could be interpreted as referring to setting books in general as opposed to Magic settings specifically.
(timestamp 1 hour 2 minutes)
"How was Ravnica recieved?... Ravnica by the way has been received really well, I can tell you it has been one of the best-selling adventures on D&D Beyond. Fastest selling for sure. So that's cool, we've seen great success around the thing, which we deem as being well-received. So will we be doing more? Yes you will be seeing new settings, I don't know if we'll be doing any new settings this year because of schedules, but yes we are committed to doing it. The problem is we wanted to wait and see how Ravnica was received before we committed to which, and because of its late time release on there, we couldn't like go "Yes that's awesome, put out a new one in six months," so we will be doing more but maybe not this year."
The word "Magic" actually isn't said once in that entire transcript, and although the question is about Ravnica's success, Nathan took that question and answered it like it was about setting books. I find his question of timeline especially interesting, as he says they weren't able to do a new setting in six months but were thinking about doing one that year (this was said in January, so this year 2019), which I think means at that time they were deciding whether or not to release Eberron's Last War book in 2019 (which they eventually decided to do).
That said, that doesn't mean that the next setting won't be a Magic setting. All this says is that we should expect more setting books; whether it is Planescape, something from Magic, or something else seems up-in-the-air.