Green: yes, Orange: conditional, Red: no
10) We love the king day - it's really easy to cheese this. You just go and trade for it as soon as it pops. That's not really a meaningful interaction, IMO, and if you've already got access to all the luxuries, you don't get the option of the bonus. If it returns, it needs to be in a different form
9) Diplomats/Spies - Yes, and the espionage game needs to be more fleshed out
8) Unique great people - For the right civ, sure. In Civ 6, it works for Colombia, but I'd rather see the great people concept reworked more generally anyway, especially with the cultural ones.
7) Scenarios - I never play them
6) Slavery - I have no great need for this. If they radically changed their vision towards realism and included all sorts of adult themes, war crimes, atrocities, police state societies, nuclear holocaust, etc (Alpha Centauri had some of these as options and I absolutely loved that game), it could work. Devil's in the details here, but honestly I think it would take a brave... fearless publisher to do such a thing in the modern political climate.
5) Attack and defense values. Some unique units have these. Ranged and siege units have these. I don't see any great need to complicate it.
4) More options when conquering cities. Yes. Puppets are a good idea, but they'd depend on a competent city management AI. I'd also like the idea of resettling to deal with situations where the AI forward settles you. Now, you solve the problem by razing their city and building your own proper one on top of it's ashes, but that comes with a huge diplomacy penalty. It'd be better to remove the city and send the peasants back to their own empire for less of a diplomatic penalty.
3) Plague. Natural disasters were a good addition in Civ 6. Does plague need to be added? Only if the counter-play is interesting. If it's just build X building to negate the mechanic or research X tech to negate the mechanic, no. I really like Civ 6's idea of making disaster's a risk. Yes, they trash your city, but you get better yields from it after you recover. I also like knowing in advance where the at-risk areas are so you can settle appropriately, like we get with volcanoes and flood plains. If it's an interesting strategic element, add it. Otherwise, no need.
2) Vassalage. Some way to mitigate the warmonger's dilemma is a good thing. If you let them live, they chain denounce for the rest of the game, which is annoying and pointless. If you don't want that, you have to wipe them off the map, which is annoying and pointless. Being able to seize their capital, assert dominance, and move on is a good thing.
1) National wonders. Civ 6 has them by a different name. They're the government plaza and diplomatic quarter buildings. I don't think there needs to be two such districts. I do like the choices of the tiers in the government plaza.