What functionality do you want your smartphone to have that it presently does not have?

to fit in a hand

Ideally also in a pocket
- women gave up on pockets 😅
Fully agree, I really miss the mini smartphones.

Proper physical keyboard.

Week-long battery life.

Bezels (so you can touch the damn thing without operating it).

Root (in case that's not obvious and implicit).

Privacy. No listening in to send me ads.

The capacity to easily switch operating systems.

If it's a mobile computer, then I want the same operating system flexibility that I have for my PC.

Pinch to zoom anywhere.

I'd like to to tell me exactly what data it is sharing with apps. Not a list of permissions, I want the JSON

I want the apps back from 10 years ago. Well functioning, beautiful, no ads, no subscriptions, just one payment.

Fuck, I'd take the apps from last year. Google fucked up the fucking weather app over the last year. How do you do that? They had something great with the Pixel Weather app with a great convenient layout and the cute froggy mascot. Why dump that for a lifeless app with worse layout?
And Maps removed the ability to pause & skip forward media with a single easy press. They literally chose to make it less safe while driving. Plus removed navigation summaries like the distance and average speed of a trip.
Go back further and you definitely get even better though. Fewer subscriptions. No AI crap. Remember when Google Now put useful information right at your fingertips? Before they killed that for just a news feed?

I would love to have the ability to expand a reason for why the app need access to things like calls and contacts.

Does it need access to calls so it can mute itself? That is different than needing access to calls for some other reason. Maybe the reasons are a lie, but if they take the time to put in something that makes some kind of sense at least I'm not second guessing why they need some seemingly random thing.

Also it would be great if the access was more obviously granular. Let them ask for the whole thing or specific parts, because if they pick specific parts that is a sign that they are actually using it for the stated purpose.

I already have these but most people don't: a headphone jack and micro SD card slot.
Removable battery. Loved this on my old LG G4, even though it wasn't the best implementation. I really miss the days before smartphones where I could buy an external charger and just swap batteries as-needed instead of needing my phone to be tethered. I know this has largely been solved by longer battery lives, faster charging speeds, wireless charging, battery banks, and better charger availability. But I still miss the days of just swapping. Great for traveling, great for breathing new life into an old device.
Front facing speakers. My ears are not behind the device or below the device. The technology of vibrating the screen to use to as a speaker sounds terrible. My first smartphone was the HTC One M8, and it is still the best-sounding phone i have ever had. I will happily deal with the bars at the top and bottom of the screen to make this happen.
More permissions controls. Let me not just turn off my location data, but use a dummy location like the North Pole for apps that "require" it where I don't actually wan to give it uo, for example
Better support for launchers, icons, and other customization. This is stuff I already DO on all of my phones, but it always feels like I'm fighting against the manufacture to sneak it by them. Especially replacing icons, which I like to do seasonally. If I have to manually override an icon, the system seems to permanently disconnect that app from ever changing as I change the system icon set, meaning I need to manually go in and change all the ones that aren't ubiquitous every time.
Physical buttons. Bring them back. I am willing to compromise for the keyboard, but give me a physical Back, Menu, and Home button.
Phones designed for 1-handed use. I'm an adult man who apparently has medium-large hands, and yet most smarthphones are too big for me to use one-handed. I'm a musician who plays extended range guitars and basses, so I'm probably experiencing this problem less than most people and it's still really annoying. If I really want a bigger screen that's what tablets are for.
Thicker phones. This can somewhat be helped by cases, but we reached the point a while back where phones are too slim to hold comfortably too. Keeping your fingers pinched that close together for extended periods is just unnatural. It's the same reason the Switch's JoyCons are extremely uncomfortable for anyone with larger than tiny hands.

For 3. there's this app for Android which I've used before successfully FakeGPS
You can select a mock location app in the android developer options

1 a bezel on the top edge with the camera in it, instead of the screen going around and having to try to scroll to read the word the camera is covering

2 analog audio and an sd card slot

3 fingerprint reader on the back of the phone

4 removable battery

That first one don't bother me too much. I'm with you on the rest.

Micro SD slot

I would like the ability to falsely report my location to some apps while leaving it accessible for others.

Id like to be teleported to the nearest pokestop

A couple of times, I've thought "it always just seems to be improving cameras or making phones thinner that get billed in updates", but then I wondered what the things are that I would like to see myself.

I'm kind of trying to move more of my use to a laptop on Linux from a phone on Android --- more open system. But if something were going on the phone:

I'd like to have a way to get an external antenna that I could place somewhere in some situations to get better reception. Maybe have a standard for an entire external USB or Bluetooth transceiver and just have the internal SIM do auth.
I'd like external USB-connected power banks to be able to report battery level and have a phone treat the power as contributing to its remaining running time. I went through the USB spec some time back, and USB permits for power supply devices to report a battery level. I've seen Bluetooth earpieces that do report battery level, though USB power banks seem not to do so. Part of that is probably chicken-and-egg, though: unless Android (and GNU/Linux, which is in the same boat) make useful use of that data, battery bank manufacturers probably won't start reporting it. Someone has to take the first step.

Most of the time what I want is actually a better camera.
(I have better cameras, they're just not always at hand)

Most things people mentioned here, plus: the ability to buy separated components with different specs, and build your smartphone at home. Like a PC.

(I'm aware this will probably never happen.)

I love my S23, it's not too big and fast enough.

The one thing I miss is an audio jack.

Would be cool to have better window management :3.. I considered putting linux on it with a TWM but there's some android only apps I can't give up

I dunno about iOS, but there's that split screen thing on Android, if you haven't used it.
kagis
https://www.groovypost.com/howto/split-screen-on-android/
I've found it to be of limited use on phones, because the screen is small to start with, but I've used it on Android tablets.
If you don't care about the phone form factor (or don't care about power usage on the existing hardware and can do GNU/Linux on it), you can run Android apps on Linux on Waydroid, though I've never heard of someone doing it on a phone, myself. I don't know if resource usage makes that impractical.
- Unfortunately the split screen feature that comes with android is a lot more limited than what I'd ideally want.. at least on the phones I had :3.. though being able to swipe on the bottom bar to switch between open apps sorta emulates the idea of workspaces, but again.. it's not the greatest
  I have seen people run waydroid on phones :3 I haven't tried yet, but I do want to install linux on one of my old phones to try at some point, I saw someone using Hyprland on a phone which is pretty cool