In FTB there are many new possibilities as to what you could build. With new blocks you can make new structures and buildings. The following is a list of building ideas.
BANK/STORAGE UNIT Using personal safes people could find a way to safely store their items. You could make a profit by charging fees. Also you could invest in the values of certain goods, as their demands rise and fall.
BEE HOTELS Apiaries can be viewed as bee houses. By using this comparison you could make bee hotels or bee villages or even bee cities. You could even connect apiaries with little paths or decorate the area with potted plants or little streams
FACTORY Unless you are playing with a mod pack such as MagicWorld then it is most likely that you will need a factory or manufacturing plant. Mods such as IndustrialCraft, BuildCraft, and Factorization add machines to the game that help progress grow exponentially. Your factories could produce items that you need like ingots or other basic materials, or something far more advanced like circuits or even a power plant.
FARM FTB adds new types of food and other agricultural products to the game. Farms could produce simple products like wheat or potatoes, or some new advanced crops like humus or peat. The possibilities are endless. After all you can't feed the beast without food!
TOWN With Feed The Beast towns can have more buildings. Feed the beast also adds elements that could replicate modern city features such as lampposts. With more items in feed the beast naming your town can be much easier.
Thaumcraft Base- Made in a flatland/standard terrain meadows mystcraft age, this base would be built around a normal node, encased in a sphere (As close to one as possible) of warded glass. This would not be open to the sun, however, as it would be used to try out spawning various things by manipulating flux. (I-E, a witch spawner by overloading a cauldron with instrumentum, found in flint.) You may also use this design to use a node as a cheap beacon. Possible materials include warded stone, warded glass, arcane wood or stone blocks, etc.