All I want is a pleasant, well-lit, warmly heated room with plentiful storage, attractive, easy-to-clean surfaces and a nice long waist-high counter. In that counter would be a large basin along the lines of a laundry sink with a high extendible faucet that has some kind of built-in temperature regulation to ensure an evenly heated spray.
This would be used to give the baby a bath. Not stooped over in a bathtub. Not in the kitchen sink.
10,000 book liibrary, complete with spiral staircase, little special collections rooms, and a secret door to a secret room. Big tables for study and books, and working manual typewriters, as well a display cases for fountain pens and inks. Space for notebooks, handmade paper, and the like.
Also, a meditation room in a tower with a 360 degree view.
And a room with design reminiscent of traditional Japanese ryokan. Off to the side is a hot tub for soaking, designed to look like a hot spring. Also a Finnish sauna, design harmonizes nicely with the Japanese ryokan. And a cold plunge pool, easily accessible from both sun and hot spring. Tea ceremony and massage both available.
With 380 million, I could have all three, right? And still money left over to donate to various worthy causes.
First, the house must overlook an ocean. No, I don’t care how big and impressive it is, a LAKE will not work.
Then, a Camera Obscura
The link is a small site; please do not swamp it by spreading the news.
Whenever I took people to that place, they looked at me funny for suggesting it; then I had to pry them away from the screen.
Hints:
You will need a first-surface mirror and “condenser” type lenses to focus the image.
The mirrors are used in military vehicles for optical sights. The lenses have to be super fast - there isn’t much light to lose. The screen is the big question mark.
OK Dopers: you’re such geniuses, come up with a design.
This. Maybe NSFW.
I would say workshop except I couldn’t restrict it to one room. You need a metalworking area, a woodworking area, a spraybooth/wet area, an electronics/computers area, a general mechanical area and probably a few more areas I haven’t thought of yet. And they need to be mutually exclusive to at least some extent. Welding and grinding produce 'orrible spatter and sparks and grinding dust which don’t go well with wood or paint or electronics, for example.
So not so much a room as a multi-room workshop.
It’s going to take a warehouse sized building with some areas sectioned off. Even in the machine shop you don’t want to weld and grind around the completed machines you built. And then you need to isolate the huge Tesla coils also. I’d probably need an entire industrial park before I was done.
Entertainment.
I already have one but I’d love the cash to really go nuts.
Pool table, bar, pinball machines, old fashioned table based video games, a jukebox, a wall of flat screen TV’s all on different channels or able to be viewed as a single monitor.
I’m not shallow. 
A prudent and conservative financial advisor surrounded by enormous mounds of cocaine.
Nothing. A large room with no corners, in a pleasant neutral grey devoted to emptiness and void. To remind me that stuff owns the owner.
Drugs. I’d have a safe room where I had hundreds of recreational drugs (prescription, designer, scheduled, etc) as well as endless drugs to counteract the bad side effects or end bad trips to pick from.
I’m sure I’d have an awesome easy chair, a great mattress, great sound system, a great projection TV and other things to help me relax.
Also hookers.
Yes I am a stereotype.
a wood shop
The freshly severed heads of mine enemies.
As a frustrated model railroad enthusiast I would ask for a 60 ft. by 16 ft. room with one door, no windows, no posts and plenty of electrical outlets. The sole purpose of this room would be to construct a model railroad.
Edit: Kind of sad now. I have the financial resources to do this. But at my age I no longer have the necessary lifespan to pull it off the way I would want.
Endless supply of cash?
An amazing machine shop, with a fancy CNC mill that can make an engine block from a lump of metal, as well as some of the finer examples of the tools from the American post-war machine tool industry. A couple of Bridgeports, you know…so I could keep the vise mounted on one for small work and use the other for larger work. All the options of course, like power feed and DRO on all three axis.
Surface grinder, some awesome lathes–from a tiny watchmaker’s lathe to something that could turn a car driveshaft.
And if the room is large enough there would be a pretty decent array of woodworking machinery on the side, with a good dust collector system.
But if the room can’t fit all that then the metalworking machinery takes priority.
Mine would be a music room. And I would get a couple more guitars to go in it.
I’d have a whole wing dedicated to foreign language versions of stage musicals.
I’m thinking a room dedicated to all the ‘Clowns in Woods’ memorabilia I can lay my hands on. You know like a room with display cases filled with Clowns in Woods commemorative drinking cups, lawn decorations, posters. Maybe big statues or mannequins of memorable Clowns in Woods with placards telling their stories ‘Tinky Dink’ once creeped out the residents of Crabapple Cove for over 15 weeks before getting caught loitering near the middle school’.
That or a room devoted to popcorn poppers. Popcorn is yummy. Maybe a room with an old school movie theater popcorn popper operating 24/7 and lots of recliner chairs with bowls next to them where one could sit there and eat popcorn. Oh, and I like harmonicas too. I could make it a ‘popcorn and harmonicas’ themed room. You know, with multiple shelves conveniently located around the room filled with various harmonicas and one could play harmonica between eating handfuls of popcorn.
There are many possibilities, but my main focus, however, would be to set up the room consistent with these sorts of practical, utilitarian themes.
Indoor lake for swimming. Not a pool, that’s too boring. A lake, with sandy shores, no fish or weeds, plant life around it. Maybe with openings around the ceiling for birds and ducks to fly in.
A sewing room with space for all my threads to be displayed easily, a large map desk to hold all my fabrics, a huge comfy chair or two, quality task lighting. A large table for framing finished works, with a closet full of frames/mats just waiting to be used.
My kitchen would have multiple pantries - one for foodstuffs, one for electrical equipment, one for pots/pans/bowls and such. No more digging around in cupboards.
A bowling alley (four lanes) with fully automated oil/pattern machines. And the alley could be converted to a tennis court at the flip of a switch. With a robot that would retrieve all of the tennis balls. And built-in Hawk-Eye and an instant replay system.
A “holodeck” or as close to one current tech can provide. Which would basically mean all four walls, ceiling and floor would be HUGE display screens. From their you could just basically download whatever scenery you liked. Whether it be CGI or real places.