1) and this the top by a long shot: Maya unique quarter. This thing is an absolute powerhouse and it is practically game breaking. For those not familiar, any settlement with this quarter gets a 15% production boost of the cost of each completed tech. This lasts the entire game. Not to mentioned the individual buildings themselves are pretty damn good. You end up just blowing through the production queue in any city that has this. The Maya are S+++ tier right now.

The rest are not in any order.

2) Modern age - Democracy specialists happiness boost. +4 happiness per specialist eliminates pretty much the only real drawback to placing them down. Gold and food have yet to ever be a limiting factor in any of my games.

3) Camels - Camels are a resource worth fighting wars over lol maybe have them add plus +1 slot and themselves extend trade range or something.

4) Initiative - First commander upgrade of the assault tree. Lets units move after deployment. Personally I don’t see any reason this shouldn’t always be the first upgrade you take unless you’re building commanders solely to sit on a settlement to boost a city. This should be moved down the tree.

5) Embarked unit water movement. They move too quickly on water. They pretty much match naval movement and speed should be a big advantage for actual naval combat units.

6) Missionaries in general - why would I ever build a scout again? Scouts are cheaper sure but they can’t cross borders. If you want to explore the world just send missionaries. I guess that mirrors real life lol still I liked the recon upgrade path in civ 6 where they became explorers/special forces.

I’m sure there is a lot more, these are just some of my observations