Civ Vi is pretty stuffed with features already but I would buy a new dlc that expands the diplomacy and government mechanics. I particularly miss a few things from Civ V. Pledging to protect city states and negotiating peace deals with the AI to stop attacking them. I also miss Military states constantly gifting me units so I don't have to bother building up an army I also miss ideology. It was a pretty fun mechanic and it made late game diplomacy a game changer. At the moment I still find diplomacy a bit flat in 6. They either love you unconditionally or hate you but don't bother to go to war and instead swallow up city states. Other than keeping an eye on a neighbour with a big army I pay little attention to what the AI are doing.