Now I finally had time to justify my choices:
-Archeology/Archaeological Sites: something fun to have at the end of the game and I'd keep in future editions of the game.
-Barbarian clans: can be fun if well implemented. I'd particularly like each clan to have its own identity (peaceful or aggressive) that could later develop into city-states depending on the players' interaction with them.
-Corporations/Guilds, global market and economic crises: perhaps one of the most requested mechanics around here. Also, corporations can be quite interesting for the late game. Please, bring them on with an Economic Victory mechanic.
-Cultural identity/Nationality: perhaps the substitute for loyalty? A mechanic of multicultural and multiethnic empires is must for the game.
-Districts: they're fun, but I wish they were more visually connected to each other.
-Ideologies, civil wars and revolutions: completely necessary for the game. Absolutely one of the best things about Civ5 that was taken from Civ6, making the late game quite boring right now.
-Minor civilizations: imo, the game needs a level between playable civilizations and city-states. Here would be great civilizations that were not included as playable civs. It's kind of weird that big empires come into play just like a city-state.
-Natural disasters: the best thing implemented in GS and one of the best things about Civ6 overall. Natural disasters add a unique flavor to the game and map. However, please, take out Global Warming. The way weather changes are represented in the game is deplorable - it happens too early, effects are overly disastrous, the AI isn't able to handle it, and lastly, those flood barriers are horrible on the map.
-Policy cards: I like this, but there are a lot of things here that should be rethought and improved.
-Random events: if implemented well, it can make the game a lot more fun, especially for the late game.
-Religious reforms and religious syncretism: religion as a whole needs further detailing and deepening.
-Vassal states/Puppet cities: high need for the game. I'd argue that Domination Victory should not be based on conquering other capitals, but making all the world's empires your vassal states.
Honorable mentions:
-Appeal, seaside resorts and national parks: good idea but poorly implemented. Appeal is a loose end that interacts little with other mechanics and national parks have very strict rules.
-Energy/Pollution: energy can be interesting if it comes as something capable of bringing about great transformations in the match. In Civ6, energy is totally bland and you can completely ignore it without taking major damage.
-Eurekas and inspirations: they seem to be unpopular given the poll results so far. I particularly like them.
-Heath, pests and diseases: it might be interesting, but I don't think it's a high need for the game.
-Migration movements: can be fun, especially if aligned with economic crises, natural disasters and civil wars.
-More types of cultural expressions: these are things I'd like to see in the game, but difficult to implement.
-Navigable rivers: it would make city building more fun, as well as create more interactions with the map.
-New types of Great People: at least Great Statesmen and Great Philosophers would be welcome.
-Tourism: should return, but not as the main fuel for the Cultural Victory, but as a modifier of cultural influence of your empire.
Mechanics I forgot to put in the poll:
-Agendas: I don't particularly like this. It makes leaders act like fools obsessed with something.
-Emergencies: they are cool, but their prizes should be more impactful.
-Housing: almost nothing impactful. I always forget this is in the game.