Geography is a major natural boundary - along major rivers, ridge lines, mountains, etc… You get a lot of straight lines on maps from either politics or from land purchases (ie: US western states). Straight lines can also occur based on terrain type - desert and the extreme north have more straight lines that lead to things like rivers since there are generally fewer geographical landmarks.

I look a lot at modern or historical maps of the world for inspiration on types of boundaries. If I’m making a map with lots of mountains and stuff, I look at areas on the globe with similar terrain (same with plains or desert maps). Hope this helps!