I would be more interested in a built-in map saving and loading feature for developers; I would generate all of my auto-joins at run time and then save them directly to the map file that gets compiled (and write logic to stop generating them if icon_state differs from the default). My saves would work because each turf's icon state is the only information that describes the state of the autotile anyway!
So in the big picture, we need map saving and loading a little more importantly than map-editor auto-tiling... because that feature would be so much more powerful.
I came up with a chunk saving / loading thing that allows for like 30 million tile worlds. It takes a little bit of time to set world.max_ to those values, but after that, its smooth sailing. When something changes position, the chunk system is notified and it will determine if a chunk needs to be loaded or not. We also tell the check system when items are taken, added, deleted, or moved so that it can react accordingly. Chunks are only saved when they are made dirty, and chunks are only loaded from disk once (if they are about to be seen).
There's several problems with being able to save turf data quickly though, and currently the fastest way is just with text while keeping the amount of data you want as minimal as possible (bare minimum location and type), generate or process the rest.
Would you agree this is the best approach for single server worlds with what we have right now?
f << "x=;y=;z=;t="//x,y,z = loc//t = type