I don't think we will get a Civ Spin-off Game for Civ VI. Beyond Earth came 4 Years after the release of Civ V, Colinisation 3 Years after Civ IV. If we would get a Spin-Off Game for Civ 6 this Year, then that would be 6 Years since Civ VI came out, that's more so unlikely because all the Spin-Offs so far came 1 Year after the 2nd Expansion of their Game, which would mean a Spin-Off Game for Civ VI should have been released instead of NFP, or at latest in 2021 considering Corona. I think Firaxis may have considered making a Spin-Off Game but decided for NFP instead, which is without doubt the better decision.
Besides, IF we would get a Spin-Off this Year, then that would mean no Civ 7 for at least 2 more Years, till 2024 at most. I highly doubt that, and I would be very disappointed if Firaxis introduced all those Fantasy/Sci-fi Game Modes in NFP just to make a Spin-Off to introduce more of that.