I've made small Nether bases in the past, but focused mostly on expanding my underwater colonies. For once I thought it might be cool to focus on the Nether, building whole cities designed around the unique dangers of that biome (building interiors too small for Ghast to spawn inside of, enclosed arboretums, vertical farms, etc.)
There's also a side project: Terraforming the Nether. At the same time that I expand the Nether colony, I also layer soil in a spot I call the Oasis and spread grass, plant flowers and trees, and gradually expand it outwards. The goal being to transform as much of the Nether as I can into the overworld.