First of all, I do love Photo and use it for 99% of image editing. So, my wish list is to eliminate that last 1%:
If I've a copied image on the clipboard and create a new image in Photo, I'd like the created image to be sized to match the copied image - I often copy an image from the internet and need to edit it, but have to crop or increase the "new" image size to make it fit.
I'd like to be able to set the image size without changing the number of pixels. I've tried to use Photo to plan our sitting room furniture, with PS I could set the image size to be the same as the actual room - obviously, the PPI is pretty low(!), but it makes measuring furniture and fitting it on to the plan harder if I'm having to do scaling calculations.
The shadow/highlight tool doesn't seem to do as good as job as PS in lifting the shadows in backlit images.
Finally(!), I'd love Photo to be able to import from a scanner; I understand Windows makes this harder than it should be, but other programs do support it.