I can see how this would pose a major technical challenge, but somehow being able to have layers with different tile sizes represented all together in one layer stack (and still be editable) would be amazing.
And this feature may go hand in my hand with being able to tell Pro Motion to NOT use certain layers for tiles - the Tile Set panel would ignore them.
This would allow game artists to create their game art in context more naturally. They’d be able to see more accurately what their differently sized tiles will look like together in-game without needing to constantly repeat the monotony of exporting all the assets from Pro Motion, update game files, re-run the game, etc.
The Tile Set panel might only display tiles for the currently selected layer or maybe it lets you choose the tiles from which layers it displays, though you wouldn’t want to pick/place 16x16 tiles into a 24x24 tile grid and so forth.
Frame dimensions would probably need to be a multiple of the smallest tile grid. The far right tile column and bottom tile row of larger tile grids would either get cropped or not get drawn, since partial tiles doesn’t really make sense.
And if there is different tile grid sizes in the same file you would possibly want to be able to offset each layer’s tile grid manually rather than 0,0 always be the top left of the frame.
Then there’s the matter of the zoom grid changing per layer.
Very complicated for sure. Perhaps not worth the trouble.