My spec list would be:
Either hooks to hang stuff vertically or the stands that hold one bike at head height above another at floor height.
Security for the bikes, things like ground anchors, etc. But nothing that obvious from the outside.
Other hooks for hanging wheels and perhaps hanging wet clothes to dry.
2 works stands. A big, heavy duty one, perhaps anchored to the floor for normal usage, and a smaller one to move around as needed.
A good, sturdy work bench, with a clamp attached and ideally a wheel truing stand.
Lots of storage. Cupboards, racking, shelving, you name it. Old kitchen units work well and the work bench an be incorporated into these.
Lots of power points, dotted around the whole space.
WiFi.
A Fridge.
A tool chest on wheels.
Good lighting and/or windows.
Somewhere for the turbo trainer and turbo bike to live. A fan close by and either a permanent screen or somewhere to stand your iPad.
Bluetooth speaker.