Id really like to see some features that existed on the N900, that I thought really helped make the phone what it was, brought back on a flagship device.
Specifically, I'd love to see a flick switch that toggles the phone lock.
Also, a camera lens cover that opens the camera app when opened.
A 3.5mm headphone jack..... =(
Finally, I'd really like a return of the built in stylus. Obviously now, it would be a pseudo electronic companion device as oppose to the hunk of plastic that it was on the N900. I've always considered an integrated stylus to be a strong feature that has nearly made be buy a device, even though I didn't like much else about the device (didn't like the brand, size, launcher, etc.).
As an honorable mention, I'd of course love to see a slide out keyboard, but I know that is incredibly unlikely. So I'd be happy enough with any of the above features.
Most of your suggestions have already been made before but I think some are new:
Slide toggle for unlock - yes please, less prone to accidental pressing than the usual unlock/power button. However with fingerprint scanners we shouldn't need either.
Camera lens cover.. I prefer a separate camera button that turns the camera on when you hold it down. A camera lens cover would be useful for other things, like keeping the lens clean!
Stylus... maybe, I don't know because I never used one.