Coming to this late, and I’ll probably repeat what has already been said, but…
I’d be happy with a slightly larger screen, but I think the actual phone size should be unchanged. Perfect pocket size as it is.
On that subject, the ergonomic aspect needs work. I love the aluminium back, but the smooth, square edges make the FP unusually difficult to use one-handed. Maybe some kind of matt plastic edge is required?
I see no reason to try making the phone weigh less. As someone pointed out, this generally implies more use of plastics, and I prefer a bit of heft in my hand.
Increased screen resolution: not a deal-breaker for me, as I’m happy with the current performance (and price) so whatever. However, the primary concern must be battery efficiency.
(And yes to a wider range of screen brightness)
I’ve also had the problem of my FP turning off in my pocket. Maybe just need a longer button-press to call up the reboot menu.
Now, I may be alone this one…
In an age where hardware buttons are disappearing, I’d like to have more. You see, I still use an iPod (shuffle) every day and have never liked using a phone for music. Why? Well, if I walk into a shop and quickly need to pause a podcast at a specific point, or if I need to change track/podcast while walking around, I just do it by feel. On a touchscreen smartphone, I have to take out the phone, unlock the screen, navigate to the music app. Even one customizable button on the rim would make a huge difference for me.