While looking at an empty project in the built-in pico-8, I thought about the default map/sprite options available without extra code.

There are the two classic approaches :
1) use the shared map/sprite area for sprites.
We get 256 tiles with 8bit flags, and a map 2 screens tall and 8 screens wide.

2) use the shared area for map.
We get 128 tiles, and a 4 screen tall and 8 screens wide map.

We can also use a hybrid of the two, where the bottom of the shared space is used for sprites, while keeping the map rectangular...

Option 1 is efficient, IE every bit counts, while option 2 is pretty wasteful :
in the map, every tile index is in the 0-127 range, but still uses one byte. That's 20% of the map data forced to be zero.
Flags for tiles 128-255 are also unused...

What if these unused spaces had had significance ?
For example, the tiles 128-255 could have been a horizontal mirrors of the tiles 0-127.
Maybe instead, two of the flags of each tile in the 128-255 range could be used as horizontal and vertical mirror ? That would save a few tiles, but in tile sets, you often have the 4 versions (normal, HFLIP VFLIP H&VFLIP), or even eight if 90° rotation is also needed.
Having the referenced tile be a fixed index-128 would only allow one of the 8 variations.
Maybe have another flag : false for index-128 reference, and true for index-1.
That would mean 1 flag for reference, 1 flag for HFLIP, 1 flag for VFLIP, and maybe one last for 90° rotation... Leaving 4 or 5 flags for normal usage.

Moving the sprite data to high memory and drawing the tiles in the range 128-255 based on the flags in _init wouldn't be too hard and would take almost no tokens (probably <100) , but the map drawing in pico-8 would be absolutely miserable, with visual garbage all over the map... And if we had to use an external tool, we'd be better off with and advanced system like picomap anyway...

It would only make sense if someone reprogrammed the map/sprite/flag editor part of pico-8 in pico-8, so the new flag interpretation/map display could be added.

Has anyone tried to do that ?