Nere's Inland Flooding, Surprise Climate Change, and Renewable Energy Complexity as a combination. There's a version of it with extra unique units, but I prefer the generic-only one so that modded civs aren't left out. My favourite part is that it also significantly buffs anticav against cavalry. It feels like an incredibly natural part of the game to the point where it feels weird playing without it. Steel & Thunder fills out the unit trees nicely and balances the existing ones really effectively.