-Liquid storage towers. For bonus points, set up portable tanks and use TiC faucets to drain into them. It'll look nice
-Large power plants. Use scaffolding, iron tanks, and exposed wiring to make it look industrial
-Huge warehouse style storage for excess items, with option production of everything you can make from cobble
-A simple aesthetic park. No function, just relaxation. This kind of stuff often gets forgotten in FtB
-Minecart subways
-For buildings that have similar function, try using those glass inclosed overpasses you see in cities sometimes. They look really nice if done well.
-IC2 shop for common materials that people dislike having to craft: circuits, machine blocks, plates, etc. You can expand this to other stuff (TE machine frames/servos), but IC2 is the mod I hear the most complaints about.
-Metals exchange. Everybody has run out of that one specific ore they needed a lot of at some point. For looks, try having tanks, faucets, casting basins, and plenty of metals in block form
-Skyscraper greenhouse
-Thaumcraft/Tinkerer crafting services. There's a lot of nice items in there, but it takes some investment to afford them
-Similarly, Extra Trees/Bees shops.
Lastly, if you're specifically looking for a city look, some little details I haven't usually seen in MC cities that are all over in the real world:
-Billboards via computercraft
-Subway stations or bus stops
-Street signs
-Man holes to wiring below the city
-Dumpsters in alleys
