What should I add to my minecraft house?

It really depends on the type of mansion you want, i personally like victorian, or gothis style masions, while i know many people like more modern styles.

for Gothic, you need large windows, with stained glass (if you have the mods, just big windows if you don't),
for Victorian, you need grant staircases, and secret passages.
for both, lots, and LOTS of torches.

for modern, it should be bright, with lots of natural light, and very space efficient (Glowstone replacing a ceiling tile, rather than torches)

perhaps have multi-level floors, like using half slabs, so the living area is down a step from the rest of the house

We'll you should obviously add bedrooms and put in there own ensuites and if it's two stories add balconies for the bedrooms. You should add a pool with a high and low diving board with some water slides. A doghouse and an ocelot house. You could add a huge library/ study, a huge kitchen and a loungeroom with a bar. A hedge maze. A basement and attic. A cinema room. A trophy room. A secret room behind the library cases. In the secret room have portals and cells full of monsters and have a firework platform. A laundry. A dining hall/room. An arcade with a bowling alley, shooting range and mini golf, etc. A soccer/ sporting field, a tennis court, a basketball court. Those are some of my ideas.