doctorbadwolf
Heretic of The Seventh Circle
Mystara and Dragonlance are both too big and would need at least their own books to do them justice. Even Nentir Vale by the end had a lot of lore behind it.
Not many D&D worlds could fit within this format the way many MtG worlds can. Maybe if you stripped Planescape down to just Sigil you could fit it in.
Zendikar has seven continents, but none seem very detailed honestly, compared to say just the Sword Coast of FR for example, or a single major Dragonlance nation.
Realistically I see this book just being MtG settings that can be functional condenced into an Planescape/Spelljammer/Ravenloft style MtG metasetting.
Maybe 20 to 50 pages per setting, not oncluding player options. You can't do any true D&D setting justice with 20 to 50 pages. Heck, they didn't even really do a Waterdeep justice with 25 pages (it should have gotten as many as BG in BG: DiA), but you could fit a lot of MtG settings into a space like that with ease.
I think you can absolutely introduce players to both worlds in the space of a “gazetteer”.