This is more or less what I am doing in my dynamic dungeons project. Unfortunately, I haven't been able to pick it back up in a long time, but it still exists as a proof of concept for this functionality. Hopefully I can finish it one day. There is a video showing it in action linked from the beginning of the article:

Sounds like an interesting idea. While I have no idea if something like this will be made for CC4 or not, you can already today get access to all the data needed, so that should be able to be done with an add-on, meaning even if it isn't put into CC4 by default, it should be possible to write for anyone with a bit of programming experience and put in without PF having to add anything to pave the way.

The technique Ralf showed here unfortunately only works with bitmap fills. This covers most maps, but some maps, especially maps using line-work (like the landform maps) use different techniques for the fills.

It gets a bit more complicated to scale these. I am pretty sure Ralf showed that off in a video at some time too, but I don't remember which off the top of my head. I'll see if I can find a link for a video or something else, unless someone bets me to it.

If you try with for example the Mike Schley overland style, you'll find that this trick works the way Ralf showed it if you use it on a bitmap-filled polygon, like farmland for example.

edit: Ralf demonstrating editing hatch styles in here: https://www.youtube.com/live/-zZGWKT2pN4?si=LOm5YLc6CtJAds-j&t=1187

For sharing, yes, it is completely fine. The only things that matters when sharing the file is that the recipient have the actual art files on disk. You can make as many fills as you want inside the map without that being a problem, sharing issues only becomes an issue if you manipulate the png's on disk (such as making copies of them). As long as you can see inside that fill style dialog that both versions point to the same png files, then everything is fine.

Finally in the atlas. Sorry for the delay.

As have been pointed out above, this starts to happen because your entities have too many nodes. But I want to point out the reason why this happens in the first place, namely too much fractalization. When you draw fractalized entities, CC3+ places intermediate nodes to be able to make this fractal shape, and each level of fractalization doubles the amount of nodes. This quickly gets out of hand if your setting is too high. When you draw large landmasses, it is adviced to use a lower level of fracatlization. This can be reduced using SIMPLYFY as mentioned above, but it is probably best to draw with less fractalization in the first place.

You can however make a drawing tool that emulates one of the basic tools. Just set it with the desired shape, and then set all properties of the tool to use "current properties". This will make it act just like a basic tool, but it will have the options like trace (provided the shape picked for the drawing tool supports it).

Yea, there is actually a white polygon around most maps by default, on the SCREEN sheet, it is used to hide stuff that might be sticking outside the map border, since a drawing is technically borderless. But the printer will still see that as a valid area to include int he printing.

What you can do to avoid it is to print just the 'active window' instead of 'everything'. Before doing that, simply zoom in to your map, and maybe resize the CC3+ window accordingly so that the view only shows your actual map, nothing more. The best way do do this is to first use the Zoom Window command to define a tight rectangle around your map, and then resize the window afterwards, because CC3+ will remember the zoom you defined and always fit that part inside the window. Also remember, CC3+ do have the map in a separate inner document window, you can resize that instead of the main CC3+ window.

For the second point, CC3+ scales to paper on print. The map itself should be scaled according to the actual real world size, not the print size, so if you wish to cover an are 500 by 500 miles, set the map dimensions to 500 by 500.

In the print dialog, you can then specify desired print scale when you print, or if you export to an image, set the desired pixel size as needed by your print.

Which ProFantasy folder are you saving to?

It sounds like you're saving to C:\Program Files (x86)\ProFantasy\CC3Plus, which is the programs installation directory which is not supposed to contain user files. In fact, Windows have protections in place to prevent users to save to that folder, one of these is folder virtualization which is the reason you may see files inside the program, but not when trying to view the same folder using file explorer. The other protection is UAC.

Personally, I don't recommend saving your maps (or exports) inside either ProFantasy folder, but rather use your documents folder, this is where user files are supposed to be kept. But if you wish to save inside the ProFantasy folder, make sure to use the data directory, not the installtion directory (c:\ProgramData\ProFantasy\CC3Plus by default)