Come to think of it, there are so many graphical enhancement mods for Morrowind, I'd bet that every asset has already been recreated by someone already. There might already be something close to a libre reimplementation of Morrowind, except for rewriting the dialog and quest scripts. Which would be a lot. Ok, close-ish, maybe.

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?