So we chose a starting resolution (1920x1080) and divided it by multiple factors looking for the most adequate one to fit a 16:9 ratio. At 4 points per pixel we found our target resolution which is 480x270.
This resolution is very close to the PSP (480x272) and PSVita (960x544), similar to extended arcade and compatible with all the modern consoles. Now one problem is the 3DS screen with its terrible vertical resolution (240), to make it compatible every other system should have its playable area height reduced by 30 pixels (270-240) and use that space for the HUD while the 3DS uses the lower screen for that same purpose. Finally we made the following graphic guide with the most used ratios and resolutions:
With that as a guide we can now focus on the playable area for every aspect ratio to achieve the best resolution compatibility avoiding pixel distortion on most systems. For example, on the image the important parts of the boss are inside the smallest rectangle, which is our effective area for enemy placement, so it shouldn’t be too hard to adapt the game for a 4:3 ratio.
The game could also be resized at will, but that means possible pixel distortion so it will be locked at 2x, 4x, etc, multipliers to avoid this issue.