BUG is used as the UI in many mods, because it is superior to BtS's UI. Of course it also has had a lot more time to be developed, and is a hobby project not subject to the constraints firaxis is under as a company. That said it makes no changes to base BtS. BtS has been around for a while now, and there are alot of improvements that have been made to the base game; some of which greatly enhance things. For instance there are now Revolutions (allows cities that get fed up with an empire's management to rebel, spawning armies and causing civil wars), or Barbarian Civs (allows barbarian cities to settle down and form entirely new civilizations), inquisitions and other new concepts available.
After playing BtS for years, you may enjoy trying some of these enhancements. And that's what the OP stated, they wanted something like an expansion pack. Legends of Revolution is exactly that, and is very stable, easy to use (it's click and play) and doesn't change much from the base game; just as much as BtS did to vanilla civ. There are also many other great mods out there. Rise of Mankind is the second most popular mod, and it adds in pretty much everything that's been created by modders. This means RoM has 5 times the techs and units as BtS, which may be too much for many users, but it's the biggest and best mega mod out there if that's what you're looking for. Also Planetfall and Dune wars are great and stable remakes of the game in a sci fi setting for those interested in sic fi. There is also the History of the Three Kingdoms mod, which is probably the best historically themed mod out there, and is every bit as interesting and is as well designed as the more well known RFC.
I guess I just don't understand why 2 posters felt the need to come in and recommend an unmodified mod component, BUG, when that's not what the OP asked for; and there are so many great mods out there. BUG is great, it's also used by many mods (LoR uses it), because it's such an improvement to the UI. But it's still just BtS; after 3 years try something new, you wol't be disappointed if you pick one of the mods I just listed, and there is always the modpacks forum to browse, which has literally hundreds of mods available, probably one of which is tailored to your interest.