I like how Daggerheart did its mini settings in the book. I'd like a book of that - all new settings & genres - given a 5-10 page treatment with a "world" overview map and local map to get things started. The Book of Worlds as it were. If any of the settings become well-liked, then they can do a full-blown book just on it.
Radiant kinda did that, but something about the presentation just made me gloss over it - maybe it was the Radiant Citadel itself, "forcing" all the mini-worlds to be linked together.