I’d like a tankless water heater. And larger, more modern bathrooms. If I ever win the lottery I’m taking a sledgehammer to the ugly '70’s pink tile in the master bath, and I will enjoy it.
I want a linen closet, but there’s no place to put one. 
A huge soaking tub (I’m tall and being fully immersed for a change would be heavenly) and a working fireplace. Oh, and hardwood floors instead of stupid carpet.
Free trash pickup.
In theory I’d like a skylight in the roof of my bedroom so we could lie in bed and watch the lightning in the sky and also hear the sound of rain.
Unfortunately, I don’t know what you’d do about light from inside the room reflecting and making the skylight a mirror over the bed, which I wouldn’t like.
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A dishwasher (other than me and my two hands)
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a kitchen bigger than a postage stamp.
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w/d hookups
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Better natural light (particularly in the bathroom)
Don’t get me wrong - I like the apartment well enough that I renewed my lease, but even one of those things would thrill me. 
More walls. Both parents were artists, and so am I. I have enough art work to fill a mansion, and not even enough walls for the best pieces. So I have to rotate.
Hook up the cistern, seriously reduce our water usage and costs.
My home in uniquely arranged so as to be perfect for solar panels, with a walk in attic, etc. I so wish they’d come down substantially in price.
We put in a woodstove, 5 yrs ago, and saved large on heating bills, similar electric and water usage reductions would be sweet as we near retirement!
I love my house (really I do), but I wish it had the following:
–a huge walk in shower instead of a bathtub with a shower. I never take baths, and I’m always afraid that I’ll trip over the edge of the tub when I get in and out of the shower.
–an upstairs linen closet.
–French doors to outside in the kitchen. And living room.
–awnings on my south and west facing windows, as well as my west facing front porch.
–a privacy fence around my entire yard. I love my neighbors, but I’d love them even more if I didn’t have to see them and them see me.
A dishwasher
Central AC
Laundry
A counter in the bathroom
A central atrium 30 stories tall, roofed with glass and papered in gold leaf. A tree house out back in an enormous oak with an elevator and indoor plumbing. A Gothic fan-vaulted nave with stained glass windows, a chapter house with pool tables, and a 5-lane bowling alley in the undercroft. A library with two-story bookshelves and rolling library ladders. A stone wine cellar reached from the secret bookcase door in the library. A big red barn with moo-cows and milkmaids wearing really cute milkmaid outfits. A missile silo filled with lager. An extreme topiary filled with dinosaur-shaped hedges the size of actual dinosaurs. A bell tower with a carillon that’s MIDI-controlled. An observatory dome with a 30-meter reflector. Minarets.
Also, a whole house fan. I miss the one from our old place, it was pretty neat.
A spare room. When my son and his girlfriend stay here, it’s cramped and unpleasant for them.
It’s already been mentioned, a screened in porch on the upper deck. But right now the hummingbirds are used to zipping through and shortcutting their way around the house. They buzz mere inches above my head when I am lounged up there amongst the trees.
A 250 gal reservoir where my rain barrels could flow into, and maybe a little trickling water fall feature as well.
What else, I would RIP out that damn dishwasher and the damn ice maker and their sweating lines making damp places behind the cupboards! Really, I would. Hand wash everything. My soulution? get rid of all but 4, ok, company’s coming, six placesettings. ONE set of pans. I’ll keep the wine/cocktale glasses. That way, the sink will never overflow the counters with dirty dishes.
What is the kitchen but a bomb shelter for the cat anyway…
Hm.
A half bath.
A spare room with its own ensuite full bathroom, and a walk in closet.
A walk in closet for our current bedroom.
A large pantry with space for a huge chest freezer.
A laundry room with the space for a full sized washer and dryer [the model that has the drawer underneath that will self add the liquids to the washing]
An arts and crafts room so the rest of the house doesn’t get cluttered.
Frankly, an entirely different house, custom built on a different plot of land somewhere other than Connecticut.
More than two square feet of usable counter space, a garbage disposal, and a dishwasher. But the location and price can’t be beat, so I manage.
A garage, or at least some sort of covered parking- especially for my motorcycle.
I would relocate the whole house to be in a quieter area. Also, I need a one-story house, or I will soon. So will my dogs.
Other than that, instant hot water and better AC upstairs. And more windows that actually open (WTF, builders?).
I hate that, I love a nice breeze, especially the first few really nice days after the snow melts. Nothing like a lovely spring day and popping open all the windows to change out the old stale air. Another reason I hate many high-rise apartments/condos.
In no particular order:
A basement of any type (for sheltering during storms).
A less-wonky living room layout.
A fence. We rent and our yard abuts the field behind our house. It’s not the shortcutting we mind so much as the parents/siblings who are observing sports practice and think our backyard is convenient when nature calls.
I wish I had central air conditioning.