Splendor: The Silk Road
Buy this firstBGG reviewers call it variety without added complexity: easy to teach to new players and it doesn't make games longer. The Cities module is the favourite.
Needs: Just the base game
Adds: Two modules: The Cities (city goals replace the race to 15 points) and The Trading Posts (tiles granting permanent bonuses once you own the right cards).
Fixes: Every game of base Splendor ends the same way, racing to 15 points; city goals give each game a different target while turns stay just as quick.