It took Civilization V two full expansions to become the classic it's regarded as today. Civilization VI is halfway there. There's lots of new stuff on offer in this expansion, from UI tweaks to new factions, but I'm not here to rattle off bullet points. I'm instead going to be talking about R&F's three main additions, and how they all work together to change the way each game of Civ VI feels.
Civilization games have all shared the same core design ideas. 2K's marketing will try and sell you on new stuff all they want, but the nuts and bolts of this game are the same as they've always been and they're what really makes Civ great. You take turns, you build cities, you research tech, you fight.