(1) PS (the version i have anyhow) does not support PNG8 with 8bit alpha. This annoys me a LOT because i have to use another program to generate these from PNG24 for all my web work. why doesn't PS support this format?? PP to have this please!
(2) Sometimes i want to quickly add some markup to an image. arrows, callouts, etc. This is a PITA with PS because it's just TOO complicated. A "simple editing" page would be cool.
(3) I do a lot of work with APNG (animated PNG) now, which appear to work in most browsers. most editors won't touch APNG. PP will load and generates each frame as a layer. would be nice to see the frame boxes and edit/move them, then recombine. would save a TON of work with APNG authoring.
(4) edit JPG/PNG metadata please.
blue sky stuff: i "waste" a lot of time in PS extracting items from images. I know this is HARD, but any help here would be a massive boost and worthy of payment. same goes for "unblurring" as alternative to sharpening. i know this is technically impossible, but it might be possible to help fix camera shake a LITTLE BIT, knowing it's shake and not just random blur, etc.
my 2c, but good luck with it!