List what a map can and can not contribute and explain why about book HOT 1 OPEN

To add a bit more context:
The limiting factor is that maps can't add anything UI-related, which is what's preventing them from adding factions (for things like flags, logos, console/sidebar chrome). I have no idea how feasible it is to change this so maps can add UI bits.

As for tiles, it is perfectly doable to make it so maps can add new tiles, the code just isn't there yet.
More from the relevant discussion on Discord:

the tileset code was never updated to support merging yaml from maps
the way artwork worked at the time stopped that from being done at the time it was done for everything else
but artwork doesn't work like that anymore

from book.