The first thing it needs is worldbuilding. Lots of it. To match what's in the civilopedia.
But are there other important things? Focus on simulating a fantasy world with magic and monsters in a 'realistic' fashion like Dwarf Fortress?
making Heroes of Might and Magic series out of Civ franchise huh?
personally battle scenes and army organizations are best implemented HoMM models (to some if not all degree).
Then again F'Xis has to license or even do worldbuildings of mythical humanoid civilizations. first F'xis has to determine whether do Elves, Dwarves or Orcs and goblins have diversified nations or existed under unified leaderships? even Elves are generally diversified into three groups (based on Dungeons and Dragons settings, which itself a cornerstone to Medieval Fantasy genres made by everybody else)
A. Wood Elves: The most common 'Elves' we usually known. they later became factory workers when Saint Nicholaus left Byzantium and resettled Norseland and christianized these peoples.
so often popularized as a race of ace archers (modelled after actual English Yeomanry, though they are hunter-gatherers rather than agrarians).
B. High Elves: Elves that develop 'civilizations' silimiar (and so often superior) to humans. they may even be scientific race or magical race entirely.
C. Dark Elves: (Drows and maybe other terms), those who are so often viewed as 'Evil'. in some settings they can be the same people as Arachnoids in Robert A. Heinlien's Starship Troopers. (Original Arachnoids in that settings were very much modelled after Dark Elves, fighting elements are in fact humanoids with hive minds, this race can also be modelled as being hitech race rather than ones modelled after Xenomorphs in Aliens settings (too bad this franchise is f___ed up by IP Mishandlings inside or outside Hollywood or even from the Wallstreet)
Though not every franchise viewed 'Dark Elves' as evil. Blizzard's Warcraft franchise redefined them as nocturnal people and even listed them as 'original' elves and pure hunter-gaterers like Wood Elves. Jon van Caneghem's era Might and Magic VIII shaped them as being more 'human' in societies (and developed the same 'human' cultures and civilizations, they however have hispanic or Native American skins) . ruled under Merchant Repubic as opposed to anyone else in the same settings that has rigid monarchy system.
All in all. these 'Elves' are modelled after J.R.R. Tolkein's fantasy fictions. and his view became standard view of Elves accepted by others.
One that's less of Worldbuilding is mythical monsters: Dragons, M-Taurs, and such, in place of, or side by side to Barbarians. these are mighty being. any player that successfully killed ones earn hefty rewards.