RekMod is an excellent balance pass and similar to the popular LekMod from original Civ5. There's also mods for the expansion rules/content; Gods & Kings and Brave New World if you prefer to stick to RAW.

Ancient Civilizations and Community Maps add a TON of civs and quality maps, though I must mention that vanilla UnCiv provides very granular control over map generation already. CM does have really sweet "pre-constructed" maps though.

DeCiv Redux and Alpha Frontier are complete overhauls and have little to nothing in common with Civ5 aside from basic gameplay. DeCiv is fun, I haven't tried Alpha Frontier yet though.

Graphics-wise there are tilesets as well as alternate unit art. I personally have 5Hex Tileset and AbsoluteUnits. Looks like some mods have FantasyHex as a dependency as well.

There's many many mods besides these, but this is the list I've personally curated over ~100ish? hrs playtime.

Did I mention there's multiplayer?