Let’s say you won a billion dollars and you chose the lump sum and, after taxes, your take home amount was about 380 million dollars.
Okay, so, let’s say you buy a new house. Or build a new house. Or have an extension added to your current one… Whatever. Suffice to say, just for shits and giggles, you’re going to dedicate an entire room just to WHAT?, simply because…hey, you’re a millionaire now. Who’s gonna tell you you can’t?
Dinosaurs. I’ve always loved fossils. That’s not a deep thought I’ve had it’s just the first thing that popped into my head.
Flags and photos of countries around the world. Over 190 flags draped on the wall. Hundreds of huge photos (i.e., 5x10 feet) illustrating the Himalayas, city skylines, etc. And photos of space and astronomy too.
Basically, a huge museum room in my house. Maybe the size of several basketball courts.
I’d have a heck of a plant room with fluorescent set-up, dedicated entirely to lesser known gesneriads. Or a citrus-and-hoya themed indoor conservatory.
I want a reasonable large dome study with a continual star field projection. Oh, and I want on microfibers, small stars suspended from the ceiling in the appropriate positions relative to the sun (center of the room).
Computer museum. With all the exhibits fully functional and software available for use.
If I want to play ADVENT on a PDP-10, by God, I will.
I’d have an entire room dedicated to a huge palette-load of cash!
It’s pretty stereotypical, but I want a library. Specifically, I want it in a tower formation with circular stairs and the bookshelves as the walls. Why? Because you’ll walk up the tower and pick out the book you want. At the top is the sitting room to read in. And when you leave to go back down, you can put the book back on the shelf as you pass by. Bonus: I get exercise by climbing up there. Con: probably not great for old age but hey…if I’m a billionaire, I’ll put in one of those chair lifts later.
I’m a simple man.
One room would be a huge movie theatre. I would watch movies and television in comformt. There would maybe ten seats, all recliners. A fridge for sodas and of course, a Starbucks Espresso machine. One of several the house.
Cats!
Now, I am a dog person. I’ve never owned cats. But I do love animals of all kinds. My house and property would be a huge sanctuary for rescued dogs, but I would find it in my heart to devote a lovely room to cats, with cat-things like cat trees and scratching posts and sunshine. Maybe a cat atrium or something.
Rescue all the animals!
Physical therapy/massage/body work, with a staff of professionals. (No, actual professionals, you pervs.)
My bigly successful Presidential campaign. The room will be yuge! Like my hands.
Stranger things have happened…
I have spent a long time fantasizing about this room. It would be a large up stairs room with all windows and a different type of view from each window. Not much decor, whatever current hobby I had going would dominate, most likely several things at once. One section of the room would have a padded floor and be dedicated to making love while rolling around in water colors, if the paint got loose and started working it s way around the room so be it.
Mine would be a fancy tack room (horse equipment) with knotty pine walls and dark green centered decor. Warm lighting. There will be all my collectible books on shelves, all my tack arranged nicely and paintings on the wall. Cozy furniture to sit and read and look at all my cool stuff. I have a pretty good start on the inventory in my garage. Five saddles and no horse. I lost my last one a few years ago and need to get another one! 
Then I read Zipper JJ’s post about a kitty room and remembered I’ve wanted to have a retirement home for unadoptable senior kitties. I’m a bazillionaire so I’ll have both rooms. The well=behaved kitties can hang out with me in the tack room.
Sigh.
Aquariums.
I would have the in-home equivalent of that immersive tank that you see in all the major places. Basically the whole room would be one giant salt-water tank with coral and fish and sharks and eels and rays and everything. In the middle of it all there would be some chairs for sitting down and reading.
I’d have an entire floor of closets: one room for pants, one for sweaters, one for tops, one for jackets, one for outerwear coats, one for jewelry, and several for shoes.
If you had asked me five years ago, I would definitely have wanted a library, something I wanted for most of my life. But I recently gave away my entire collection of books, and now my library fits in one 5" x 7" device.
I want a real Boeing 787 flight simulator, too, like airline pilots use. Maybe $10 million?
Otters for me to play with (being cared for and studied by well-compensated marine mammal professionals while I wasn’t playing with them, of course).
I’d put in a full-fledged maker space. High-quality 3D scanner and printer, laser engraver, multi-axis CNC mill, ceramic kiln, the works. Maybe even a foundry. Plus, of course, all of the standard tools like a couple of different table saws, a drill press, that sort of thing.
A library. A big, Disney-Beauty-and-the-Beast library.
I’d make one for my crafts too, but I like to do those while watching TV. There will bbe no TV in the library.
Ivylad will pimp out a woodworking space.