Consider creating an asset.AsMap() helper to simplify map operations when editing map files.

Keep in mind helpers for editing maps would also be useful when editing an already-loaded map.

I started playing around and will add to it as I get a better understanding about how everything works.
https://github.com/smokingfish/SMAPI/tree/develop/src/MarcusUndAnneMod

There is a simple extension for merging. I want to add some more content layers for other parts too.

AsMap().Merge(newMap) - works fine with simple tile edits and replacing the warps, lots of work still to do :)

I have no current plans to implement this, but I'd be interested in a pull request if it worked well.