This is the link for the Cartographer's Annual issue JimP mentioned, although as you'll see, that's intended primarily for drawing a randomly-created night sky as seen from a specific planet.

However, you could use the same procedure as in that Annual issue to construct a random basis for your galactic-scale star systems map. That was done using the Symbols In Area command. Resident mapping expert here Remy Monsen presented this as the last of his 52-week Command of the Week series of written tutorials that ran throughout 2017. You can find that article on the Forum here.

Of course, all that would do would be to place an appropriate number and type of symbol randomly over whatever area of your map you'd chosen. It would then be up to you to decide what the details were for each system (though by using different symbols, and assigning types of detail for each symbol on a separate key list, you could make a start on that process).

Symbols In Area is a useful tool in many respects, because you can vary all the random parameters as much as you like, and by drawing different areas, you can add different populations of symbols to different places. For instance, if you wanted a greater density of star systems in one place than another - along a galactic arm, say - you could just draw a suitable polygon shape wherever you wanted the arm to be, and then add a selection (or selections) of symbols to that area alone.

There are ancient examples of the legged triskelion as well, from Greek art, for instance, while such "rotary" symbols more generally go back far into the millennia BCE, commonly with three or four "arms".

I fear someone will have suggested this already, but will there be a seal with a seal on? Or even a walrus? ?

@grandula - There is a specific Marine Maps style available through the 2017 Cartographer's Annual which, although a non-pictorial modern style, might be a place to start.

Some of us have been hoping for some underwater mapping options for years for CC3+ though, so you're not alone in this as an area of interest ?️!

I keep missing days here somehow; other distractions elsewhere. So apologies if you've already covered this, and I just missed it somewhere earlier, but should there be an accessway into area 31? There seems to be a corridor linking to the level, without a doorway or secret door marker.

If you've a suitable image you can and want to use (importing an entire spiral galaxy will likely crash your computer ?), and it's available to you as a standard image file (jpg, png, bmp), you can simply create a new Sheet in your CC3+ drawing, and then import it into that using the Draw - Insert File command from the drop-down menus. Give the Sheet a recognisable name so you can find it easily when you need to turn it on and off. You may need to resize the image to get it to fit the size of your drawing properly, but that's fairly easily done, if a bit fiddly to get exactly right.

You would probably be best to also set the file image up on a separate Layer too, but that's not essential.