I dream of a fish room...
Ideally my fish room would be right behind the tank and the tank would be an in-wall tank for the room next door.
I would want a large (wide and deep) working sink, possibly several sinks linked to each other in one unit with counter space that can contain the wetness that will happen.
Definitely good ventilation of some sort (heat, AC, dehumidifier) b/c it is inevitable that there is going to be a lot of dampness/humidity. And a ceiling fan can be useful to have.
A drain in the floor would be an extra bonus for sure. While thinking of the dampness/humidity and floor drain, I suppose that having some sort of flooring that can handle a good amount of water and dampness would be something to serious think about. Regular cement can hold moisture, so making sure there is some sealant or some other sort of membrane-type flooring would be good to have.
A water station similar to the one in the image below (that I noted long ago from someone on this forum). Is it okay to post someone else's picture?
Observation and Hospital tanks -- A shelf set up with a couple 40g breeder tanks: one as an observation tank and one a hospital tank for medicating. A couple of 10g tanks, also an observation tank and a hospital tank. Also space for a shallow frag tank for new incoming corals and inverts for observation prior to going into the DT.
I'd figure out a way to have a centralized filtration system for a second frag tank, a mangrove tank and the DT. This frag tank would be growing out frags that came from the DT (as opposed to the above mentioned frag tank for new incoming coral frags that need observation prior to going into the DT).
Having the water changes automated would be a great bonus, I watched Choff set his up and it seemed to work very well for him.
Having a dedicated fridg and freezer would be a bonus.
I'd also find a way to plumb the rodi waste water out to the trees outside my house (no wasting of water), w/pump and gardening hoses, maybe even soaker hoses. I've already thought this through for my current house and RODI setup, I just need to move the RODI from the guest bathroom to the garage and get out this spring and get it done.