What should I build in Minecraft creative mode?

A replica of your house.

A giant block

A 100% accurate full re-creation of the Great Wall of China.

Start yourself a little town! You'll never run out of things to build!

Can't go wrong with a massive castle or manor house, alternatively you could build a town and surround it with a castle wall. There's lots of tutorials on YouTube about how to build various structures in a Gothic, castle-esque architectural styles.

A noose

Build anything thats in your head heheh

Build a wall.