Well with how I make images, PE at the moment isn't suited for much other than tile work for me. (It's really great at tile work though)
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One of the major things for even a single document image is the marquee support (which I know you're working on). With fully functional marquee cut/paste support and having it not bug with the tile sections then I could deal with that as that'll make things much more accessible.
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The sprite suggestion is sometimes you want to add to a tiled image a sprite. Currently you'd have to draw the the sprite but be at the mercy of the tiled functions. I figured if you had a sprite-repository like you do with a tiled one (with the repository holding the sizes of each sprite) and stored sprite locations like you do with tiles, but with absolute X/Y coordinates, it would make things like manipulating sprites on canvas easier.
The sprite edit box suggestion is because the normal canvas is tile based so you can't be expected to put a sprite on there and edit it (Unless of course you use the maruqee tool to select the area needed for the sprite and then add the selection to a sprite-repository).
The suggestion's details were mostly a convenience for sprite organization and creation within PE, but nothing necessary. More of a use for having PE also contain object/sprite locations on a layer to be exported as JSON/XML to use. (currently I just make tiles with object identifiers in them, and have the engine recognize the tile-number range)
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There are also a few minor things, like right-clicking an area that leads to the transparent BG pattern gives a black instead of transparent color. However my current workflow doesn't have layers so I'm used to having a BG color to put in to "remove" pixels so that I might have to get used to selecting the eraser. (Most of my work is in PSP 4.12, which is very ancient but more powerful than MSPaint.
Another solution is that once full copy/past functionality comes to PE is to also include a way to fill an single color area with transparency. This would actually make it so that my tools can work seamlessly together as if i need a sprite I can open the document, copy/paste it and then make the BG transparent (or add an option that deletes all of a single color in a layer or selection.)
I don't want to divert your own image of what you want to software to end up as as I'd rather have a fully fictional piece of software with a single purpose than a multi-functional one of lesser quality. Heck looking at it most of my issues can be worked around with proper marquee(In progress) and copy/paste support, or even just "Import from clipboard to layer" (although I'd prefer the ability to copy/paste in the program)