I bought my house with darkroom details in mind. It had a basement large enough – though not quite as large as yours - for what I thought would meet my needs, but still there were things that I missed. I’d certainly recommend reading up on design considerations in AA’s and Kodak’s publications. There are many many things to consider, but at this point in your work you might think about:
1. TWO rooms – one for the darkroom and another for finishing the prints, i.e., mounting/framing/storage of materials for same. Allow for a bunch of electrical outlets around the room to handle the necessary circuit loads as well as a master breaker box outside the darkroom to kill all the circuits.
2. Placement of safelights, with white walls for good visibility when moving around.
3. All plumbing visible on the finished walls, so leaks are located and repaired at minimal cost.
4. All furnishings with enough space underneath for mopping up leaks/spills.
5. Ventilation (with dust control in mind).
6. Convenient light-proofing of entry/exit door.
7. Etc. etc.