Well, let's look at history:
Civ1 came out in 1991, with colonization in 1994 and civnet in 1995
Civ2 came out in 1996 (5 years after civ1), with alpha centauri coming out in 1999
Civ3 came out in 2001 (5 years after civ2) with no spinoffs, but the play the world (2002) and conquests (2003) expansions
Civ4 came out in 2005 (5 years after civ3), with civcity: rome in 2006 and civrev coming out in 08, and the warlords in 2006 and beyond the sword in 2007
Civ5 came out in 2010 (5 years after civ4), with beyond earth and civrev2 both coming out in 2014, and gods and kings in 2012 and brave new world in 2013
Civ6 came out in 2016 (25th anniversary, 6 years after civ5), with no spinoffs yet, but with rise and fall in 2018 and gathering storm earlier this month in 2019
Civworld came out in 2011/2012, but that's free to play
So, there's a history of 5-6 years between main games, with usually 1 or 2 spinoffs before the next game is released. It's currently the third year (about 2.5 years, but I'll round up), so they very well could just release 2 spinoffs over the next couple years and call it a day. However, since they did purposely release this game in the 25th anniversary year, and we have 1 less civilization than the last game (civ5 had 43, 9 more than civ4; civ4 had 34, 3 more than civ3; civ3 had 31, 10 more than civ2; civ2 had 21, 7 more than civ1; civ1 had 14), and there are still plenty of staples not in the game yet (I went go ver this list, because that's been done multiple times in this thread), I can definitely see them releasing one more expansion pack with 8 civs, bringing the total of 50. I said that in my last post, it that's my reasoning. The next expansion would probably be released in 2020, maybe in February, since that's when they released both of the other expansions for this game.
We would then probably get a spinoff later in 2020 and another one in 2021 (either of which could be civrev3)
Civ7 could then be released either late in 2021 for the 30th anniversary (honestly, the worst of these three options) or in 2022 or 2023, if they don't feel like marketing it that way.
This would all be completely in line with what they've done in the past, and the game would still have a third expansion.