What would a revamped Civ 4 really add?
Multiplayer stability. I still have issues playing Caveman 2 Cosmos and FFH over a LAN. Eventually they go OOS and reloading doesn't help for long. Most people don't even have the patience for that nowadays.
Features included in BUG/BAT and other mods, like zoom control, GP and GG progression of the top of our screen, Ctrl-x city placement planning,
Make Chariot Archers, Horse Archers thru gunpower using Cavalry a separate path from melee using Horsemen, so the Spearmen & Pikemen's bonus won't apply to them (they are not charging, they are shooting targets at range),
Add more resources (apparently poultry was never hunted or eaten by any civilization ever),
Active terrain (FFH has storms, PIE's has sandstorms; both damage units),
Recon unit progression throughout time,
Allow siege units to attack at range instead of suiciding them,
Realism Invictus introduced a stack penalty if we have too many units in one stack. So we can have 20 units if we want, but they won't be as effective as 2 stacks of 10 because of the logistics issues of maneuvering so many people at one time over a large amount of area.
More religions (Ancient Egyptian, Ancient Greece, Roman gods, Norse gods, Celtic beliefs, etc)
In some mods our workers can get promotions making some more valuable and worth protecting,
Ships can be customized to have a skeleton crew for +1 speed but -1 str, or +1 extra cargo space but -1 speed, or +1 str but -1 cargo. These create variety in every game.
I could go on.
The base BTS is a good start, but Wow is there MORE that could be done to improve upon it.
Now, do all this and update the graphics for 4K TVs at a minimum and they'd have a game worth buying again.
Just don't put in any of that tracking software that's been giving Civilization 6 a bad name. No one wants that.