As
@Sostratus pointed out, the original BE was a natural outgrowth of the Civ V engine and assets.
I would note that another spinoff game could be delivered as a natural outgrowth of the Civ VI engine and assets, with similar savings in development costs. Whether such a game would be a good return on investment (ROI) would seem to be linked to its story, its world-building, and its choice of factions, based on most of the comments here. I purchased Galactic Civ III, dabbling in space-based 4X games, and I enjoy BERT very much. The Rising Tide expansion is more fun (for me) than the base game.
I seem to be an exception to most players here, in that I don't play the historical Civ games (3, 4, 5, 6) primarily for who their leaders are. I would be interested in playing "America" whether it was led by FDR, Lincoln, Washington, TR, or yes, even Fielding. I don't know enough Chinese history to know the difference between the leaders that have been put forward for them in the various games of the franchise (except Mao Zedong). I want the factions to have interesting characteristics, which they did in Civs 3, 4, and 5 and really ramped up in 6. Those interesting characteristics can be given to aliens, humans, genetically enhanced humans, sentient mammals (dolphins?) as well as geographically diverse groups.
IMHO, releasing a new 4X game (either in space or fantasy, licensing a universe or creating their own) based on the Civ VI mechanics would be a good use of their technology and would probably be profitable. It would keep us talking and engaged while they continue innovative work on Civ VII.