Hackability

Native Bluetooth GPS support. I do mapping for OpenStreetMap a lot and the built in GPS modules are just too bad for that. Currently I have to use an app to get my Bluetooth GPS connected. But sometimes this app fails and the built in GPS takes over and ruins my logs.

I want a phone with an ultrasonic fingerprint reader, those are apparently a niche feature nowadays. The optical readers are terrible by comparison.

Today my wish is for an internal pihole instance running on the device.

More phones like the Fairphone/Framework Laptop. Easy to repair and upgrade in a modular manner, as well as install what ever OS I choose.

An OEM FM/AM transmitter. I would love to be able to playback music directly to any old ass car. Or even use it as a very short range walkie talkie with the right program.

Oh I forgot, a clamshell folding phone but with only one screen and the other side is a physical keyboard, like a mini-laptop.

Nobody seems really know how to do a folding screen well, so why have the screen fold at all?

A full physical QWERTY keypad on my 3rd hand Blackberry Bold (in 2013) was the best thing I had seen on a cell phone. I wish full physical QWERTY keyboards would make a comeback.

I will never buy a phone that doesn't have GrapheneOS, so that limits me to Pixels.

I just want a new Droid with a slide-out keyboard

I've had every one of them until the Droid 4 and they never released a new one

One without a damn camera, I swear to God most of the price goes into having "quality cameras" I never end up using.

Headphone jack. I've used multiple LG phones before, currently using the LG V60 and seems like Sony is the only way to go in the future, unfortunately.

It's the rear fingerprint reader for me, I use the pulldown action constantly. That and stereo front-facing speakers.

A phone built somewhat like a desktop PC. All parts replaceable or even upgradable. Choose the components for your needs and budget, or buy a pre-built.
But let's get to more standard smartphone concept.
On software side, no bootloader locks, or easily unlockable bootloader (not requiring internet) and root access that could be enabled in developer settings.

On hardware side:
IR blaster - Control your AC, TV, etc.
Front-facing stereo speakers - could give a better stereo sound. And bezels aren't a big problem. Actually they can be positive as you can hold your phone better with them.
Headphone jack - Obvious. Would be cool if it had mode for composite output, but that's quite extra.
Dedicated fingerprint scanner on front - I had this on Moto G5s Plus. It allowed awesome gesture navigation, much better than using whatever on-screen.
Trackpoint - Early Androids used to have that. Would definitely be useful with large screens.
FM, HD Radio, DAB, DAB+, DVB-T2 Lite receiver - Radio is pretty useful. AM would also be nice, though you would need external antenna better than cable from earphones. I think FM radio is the minimum here.
TV Receiver - Just like with radio, except now you're saving much more on mobile data.
Camera with optical zoom - We've been there with Samsung in the past. But I understand it makes things too large and heavy.
Projector - We've been there with Samsung in the past, again. I think it was done pretty nicely on the projector part with Beam 2. It didn't even add much thickness. Just the rest of the phone was meh.

IR Blaster, Headphonejack, SD Card Slot, removable battery

Late to the thread but here's my thoughts on everything I'd like in a phone. Having just a few of these would make a huge difference in how much I'd want the phone.

Hardware:

1. Standard features like a 3.5mm jack and MicroSD slot.
2. Multiple USB ports (especially on tablets)
3. Thunderbolt port
4. Here's a simple idea: Instead of making a thin phone with a massive camera bump, you make a thick phone with the camera flush with the back, and use the extra space for a bigger battery?
5. User-replacable battery
6. Modules like the modo mods
7. Battery passthrough when charging
8. Upgradable SoC, RAM, internal storage???

Software (here lies my hopes and dreams that will never be manifested):

1. Starting off simple, a feature that lets you manually limit how much you charge your battery.
2. Manual over/underclocking controls for the CPU
3. Separate WiFi/data toggles
4. More control over how big or small icons and text is
5. Easy root access with app makers not getting all "you sus" over the fact that your device is rooted
6. No data harvesting (duh)
7. Better file management (and removing scoped storage)
8. Multiple logins to a phone (like what you can do with a desktop)
9. One app, multiple accounts
10. Just having the software take less system resources.
11. Open sourcing hardware drivers
12. Multiple floating windows like with a desktop OS
13. More than 3 split screen options
14. I'm sure there's a lot more I can't think of now

duck duck go tracker blocking that doesn't use vpn technology

3D Screen Pico Projector HDMI out Notification LED Removable battery

The first smartphone I owned was the Lobster 700TV, Which as the name suggests had a digital TV receiver built in. The screen was too small to be practical, but I've always liked quirky features like that

Plastic back and a removable/user replaceable battery.

Yeah yeah I know it "feels cheap" but honestly I LIKED when phones were made of more plastic. Now they're heavier and both sides are breakable. Like, the metal backs were fine, but apparently metal backs suck for NFC and wireless charging so instead of giving us some classy Nokia Lumia style plastic they went with glass backs.

Also I keep my phone in a case because otherwise the back is so smooth it shoots off the couch if someone walks across my apartment. It's legit only in the case to add some friction.

Headphone jack, apparently..

A swappable battery. I could buy a few batteries and never be out of power. I could replace the battery on an older phone without pretty much having to replace the phone itself.

I liked the physical toggle for silent mode on iPhones. Flip/slide-out keyboards were also kinda cool

As far as apps though... I really loved having a good programmable remote app on devices with an IR transceiver. The best one was actually on Palm of all things, and you could add custom icons to a layout and then record inputs from your actual remotes to replay.

Not really a feature of the phone, but a design theory. I want my small phone back. Not this "zenfone 9" size small. Like HTC One size small.

Make it bifold wallet sized, and ~1/2"thick.

These hugely tall phones where you can't reach the top third if the screen without dropping the thing are just annoyingly huge. Then they make them so thin they can't fit a big enough battery for a full day use.

Beyond that would like an under display fingerprint sensor. Rear and side mounted always have issues with cases for me, and make it more annoying when using the device while it's on a desk.

The ability to easily unlock bootloaders, IPS screen and removable batteries.

Really would love a dedicated camera button that acts as a soft shutter release. Or a button in that position that's programmable.

I got a Samsung flagship mostly for DeX, even tho I'm enjoying the whole thing, I really wish desktop mode on Android was more easily accessible. Like 99% of the market doesn't even use the required USB C version for it.

As well as better app compatibility when going on a big screen, a lot of android apps just don't scale well.

I loved the IR Blaster on my old Galaxy S4, was really fun to mess with the TV's in public places with it sometimes and was also pretty useful at home as well!