I'll give my feedback as someone who needed this for a game but unfortunately couldn't get it to work the way I expected, and also as a professional pixel artist with years of experience in game development.
- Support more sizes
I tried making a bus that was around 154 px wide. Even though the program says it supports 256 px, it wouldn't let me go beyond 128 px. It would also be nice to support irregular dimensions instead of forcing square sizes.
- Why does the top image have to be vertical?
The side of the bus is horizontal because the bus itself is rectangular. When I imported the roof, the bus got squashed, and it took me hours to realize that the top image was supposed to be vertical. That wasn't very intuitive.
- Allow importing the back and bottom sides
I'd personally rather draw the back side in Aseprite (I don't need the bottom side for my game). But imagine making a game where the bus flips over on the road, you would need the underside.
- Pre-existing outlines don't work very well
I can understand this one, since it's probably a difficult problem to solve.
- The eraser tool feels buggy
The brush is the only tool that worked reliably for me. I also struggled with the selection tool. It was difficult to understand where to drag, even when using the corner handles. The mouse cursor representation wasn't very intuitive either.
- The UI needs a proper menu
There should be a File menu and other standard menus for importing images instead of relying only on dragging files into the center of the screen. Also, I can't drag the window around in Windows because there isn't a title bar to grab.
- I'd remove the image at the top
It takes up a lot of space and feels unnecessary. Less is more, especially in a tool I would use frequently.
- Export more than 16 directions
It would be great to have the option to export more viewing angles.
- I'd focus on the core functionality first
If I were you, I'd prioritize the tool itself rather than painting features inside the program. Those can always be added later. I think the main purpose should be assembling the pieces and exporting them from multiple directions. Sprite touch-ups can be done in other software.
When painting tools are eventually expanded, the colors and palette should always be visible. Having to click the color to open a modal is poor UX. Aseprite's approach of keeping colors visible on the left side works much better.
I would also remove the entire right panel and move those options into standard menus like File, Edit, etc.
For the viewing options, I think it would be more intuitive to have a rotatable cube or directional controls similar to what 3D software uses.
That's all I can think of for now. If I remember anything else, I'll add it later. I really hope this tool improves because the concept itself is excellent, and I'm looking forward to future updates.