Some things to consider are:
Keep in mind you'll be dealing with a LOT of water and it won't always stay in the containers/tanks--so flooring should be looked at (e.g., finished wood floors may not be a great idea for a fish room).
Access to fresh water and at least one large sink that you can put a strainer into (so no fish have a chance of pipelining).
You'll need an abundance of electrial outlets and as little chance of blowing a fuse/circuit breaker as possible (if possible, get your fishroom onto an independent circuit line from the rest of your house).
Are you going to need a refrigerator for food keeping?
Lighting, what kind of overhead lights do you want (look into what is best for what you will be doing).
Heat--do you intend to keep the whole room heated or will you be individually heating each tank, or do you intend to build a barracks and have a whole water perfecting system going. If you're planning to use barracks, keep in mind that things are going to break or fail and you'll need to have fast and easy access to everything (tubing, cordings, whatever) without having to drain the tanks to be able to move them, etc.
Cool--if you live somewhere that gets blazing hot in summer, will the room stay within betta temperature ranges or will you have to have an air cooling system? You don't want your fish to boil.
Shelves, drawers, cabinets--you have or will have tons of extra things that will need storing and that you'll need to get to when you need them.
Air venting and circulating--if it's a basement (or enclosed room), you'll probably need some sort of air circulation/venting.
Backup plan--if your electricity goes out (during a storm or something), what other options will you have on hand in an emergency? Can the fish be quickly and easily moved somewhere else if necessary?
Don't know where you live but maybe think about safety--someone had their house broken into and the vandals tipped over and broke all the fish tanks so he lost all of his beautiful bettas (another reason a basement would be something to serious consider since usually an outsider cannot see what you have down there).
If you are going to sell your fish, you'll need a place to do your bagging packing.
How are you going to get all your equipment wherever your fishroom will be--if a basement, will you be carrying huge tanks, supplies, etc. through the house and down the stairs?
Personally, my dream fish room would also include a dishwasher where I can clean and sterilize only fish things, a good microscope, an office area, a fish-designated computer with internet access.....
