Android

@Rdan33l DAB Tuner!

physical keyboard. I particularly liked the two letter per button like was used on the BlackBerry Pearl.

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

Officially supported rooting! Makes this less complicated.

Niche feature....

By Apple's standards a headphone jack.

By Android's standards IR blaster. Had them in my last 2 phone and man is it nice to turn the volume down on a TV which is too loud in a public place like a Hotel Lobby or a Restaurant

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.

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

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.

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.

true background process management and battery

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

Just came up the other day, the ability to edit the name of an app on my homescreen.

Some apps that I use irregularly have names that aren't intuitive (why can't I find my video downloader? 1 hour latter Why is it named SEAL?)

Also I have both the Android App for Outlook (for my personal email), and the web app for Outlook(for my work email) and it certainly would be easier if I could name one work email.

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.

A Silent/Vibrate/Ring switch like on the OnePlus phones, headphone jack and a easily swappable battery.

This is more of a software thing, but:

The Sony Xperia XZ2 Compact (and I think some other Xperias) shipped with a 3D Creator, a 3D scanner app. I found this out after having decided on the XZ2c for other feature reasons (mainly it being smol) but as a 3D printing dabbler, this was an exciting bonus.

Was it easy to use? Kinda
Could it produce scans with enough accuracy to be useful in 3D printed part design? Maybe
Did I ever get around to using it for that? Nope
Was it fun to scan all my friends' heads with and mess around with the gimmicky facial animations? HELL YEAH

Aluminium back instead of plastic or glass

• Slim form factor
• No notch
• No camera bump, even if at a cost in camera quality.