Gameplay and total conversion mods
Communitas Enhanced Gameplay
Building on the first mod on our list, the Communitas Enhanced Gameplay mod requires the Communitas AI and Tools in order to run. CEG cuts a wide swath through Civ V's Brave New World feature list, marking changes to most aspects of the game. These include balance tweaks to religions, leaders, as well as research trees, among many others. As is the case with the AI and Tools mod, CEG is an update to the earlier gameplay mod for Gods and Kings.
Want to worry more about cave art and basic survival technology such as fire? Try out the Prehistoric Era mod for a swift boot back to the year 6000 BC, no phone booth required. It's a decidedly more primitive origin story, but one that should extend even the most epic of marathon Civ games. As a replacement for the game's ancient era, this mod should keep you occupied with things like fire eating and berserker cavemen while you wait for the years to tick by.
R.E.D. WWII Edition
This mod pulls almost zero punches in its presentation of six custom scenarios that cut across the breadth of WWII history. From the world's stage to the more intimate, but also infinitely bloody battle at Stalingrad, all the major players are present, including Adolf Hitler, Benito Mussolini, and Franklin Roosevelt. The various possible campaigns present will likely occupy WWII enthusiasts for hours.
Don't forget to install the R.E.D. WWII Data Files along with the R.E.D. WWII Edition.
A Mod of Ice and Fire
I love G.R.R. Martin's A Song of Ice and Fire. At its heart, both in the book series and in the adaptation by HBO, it's a story about dirt and water and the people who rule it (or want to). It's very primordial in its passionately violent depiction of the personal as well as political history that drives the stories people tell each other. In this way Martin's world is a perfect fit for a Civ V mod. A Mod of Ice and Fire requires the Gods and Kings expansion in order to play.
The presentation isn't perfect, and you might have some trouble selecting your favorite house to play as, but the ambition in this mod, especially in the expansive and detailed map design, is palpable. And just like in the source material, your choices have consequences.
On a technical note, be sure to disable your Brave New World DLC expansion in the main menu before launching the mod.
Strange and Historical Religions
For a game whose major expansions includes one called Gods and Kings, religion is obviously a big deal. From Muad'Dib's Jihad to the Sith Order, the Strange Religions mod has you covered. There's even a Church of Gaming. For a bit more Earthly orientation, you can check out the Historical Religions mod. As for me, I'll be meditating at the altar of Jediism.
Both mods should work with either the Gods and Kings or the Brave New world expansion.
The Elder Scrolls Civilization V Pack
When I'm looking for an experience with a little more “Fus Ro Dah” in my total conversion mod there's the Elder Scrolls Civilization V Pack. You can play as one of a variety of Tamriel societies, including leaders representing Skyrim, Morrowind, and the mysterious Dwemer.
The link above requires both Gods and Kings as well as the Brave New World expansions. For the vanilla version mod, go here.