This is all pre-1.8, so I'm sure it could look a lot nicer these days. The design aesthetic is minimalist, and based around 4-wide tunnels linking the portals together. The various sections are colour-coded with carpets, and signs name the actual locations.
I wanted something that kept the Netherrack visible given that this design is based around the ceiling of the Nether - and the contrast using stone slabs worked well. Food for thought! Look forward to seeing what you end up building :-)