I have a fairly simple one. Physical slider keyboard. Not Blackberry style.
I’m still using my Cynogen modded Samsung Relay T699 because it was the last decent phone with a real keyboard. I can’t stand Bluetooth keyboards.
I’m buying a MotoZ2 next because there is a physical Keyboard back for it.
If by some miracle you could use Moto’s attachments, I’m sure I would switch.
I’ll also echo ilo and vrata’s call for a replaceable battery.
One cool idea I would like to see is a single good quality camera on a gimbal system. Or at least able to rotate 180 degrees from front to back.
One thing I never understood about phone architecture is why apps are stuck to living in working RAM space, instead of being stored in Flash memory. I go all the way back to the C=64, where we treated the 64K as precious and offloaded everything we could to storage. I should be able to boot my phone off my MicroSD card and have all the app space available.
4K screen will probably be a must by the time this is a shipping product.
What I DON’T care about is size. I would happily hold an 8" wide and 1" deep monstrosity phablet up to my face if it meant I had all the power I needed.
Up the APP ram to 8GB, and I throw my money at you. 
That’s my $.02.