Outfit the entire studio in Monster Cable or something equivilant. It'll make a huge difference in the clarity of sound.
Look into some Googled examples of isolating the studio with floating floors and walls. Build the walls so that they aren't perpendicular to eliminate phase cancellation.
If you're using it for clients other than yourself, look at comfort like the listening area for a group of people, entertainment rooms, sleeping accomodation, ease of loading equipment, atmosphere....
Think in terms of modern recording where the mixing room is going to be used just as much as the studio itself for recording straight into the board.
Do a google of some of the more famous studios and a guy named 'Trevor Horn' at Sarm Studios. There's a web page filled with interview videos.
Maybe even check out Abbey Road Studios virtual walk through video.
The studio is old, but it gives you some ideas since it's been remodelled..