With a nice round 50 Civs and the game full of more then enough content, I think it would be best to not release any more now and go onto Civ7; otherwise the game will get too saturated with stuff based on existing mechanics. I think leaving Civ6 as it is (at least after the April rebalance patch) for a year or so will be best so we can can get some new mechancis that can't be implemented through optional game modes and the engine at present; such as map changes (altitude/elivation variation comes to mind), more indept climate change (such as desertification, oasification, plate techtonics), city mechanics (like an evolution on the districts system), better barbarians/minor-civs (an evolution on Barbarian Clans). And of course with these you will get more oppotunity to add new civilization unique abilities that you can play around with.
Although I love the new civs and leaders (and really wanted all of those that were included in the NFP in the end) we're just getting another trade civ with a modifier/yeild bonus, +1 movement, +1 vision, +1 housing and gold but no freshwater, +10% yeild in some but -15% in others, +1 great prophet point, +yeild equal to 15% of faith if X etc...
I'm not saying I will say no to another 7 new Civs for Civ6; but I think it's best to quit whilst you're ahead and on a high; leaving something in a good place, rather then going too far to the point of beating a dead horse (in this case, the engine and mechanics of Civ6) to the point where people are turning their nose up at it and start demanding actual new content and a fresh new look and feel. (And gives Civs a chance to re-invent themselves again with the new engine/mechanics)
Otherwise if we get more, I'm hoping for:
A great plains or Pacific/western USA native peoples civ
Switzerland (diplomacy, renting of units for gold, gold/banking bonuses. Can also benefit from mountains, maybe early ski resorts and hostile units take damage within Switzerland's boarders (would have to make them defensive and not expansive and offensive though))
Assyria (I really want Sennacherib's canal and irrigation achivements realised)
Nepal (I love Nepal! I think it would make a great mountain civ which is not Inca or Mapuche*. The Gurkhas would be great to represent too)
Burma/Myanmar/Pagan (although, due to current events this might not be such a good choice...)
No. The game already feels bloated.
I disagree very strongly. Civ's goal has traditionally been quality over quantity, and it should stay that way.
You cite Paradox and Humankind, but those are very different games.
I cannot agree more with everything you've said. -it is now bloated for what mechanics they can utilize and make unique, they need a fresh start and a new set of mechanics to "ease the bloating" once again and grow into.