Rather than rectangular windows, can I draw windows with rounded corners, or maybe circular windows, etc?
Or perhaps using an image like a window itself so you can dispose that "window" or hide it when you don't need it.

That way you can draw out a window you want to use in an external image editor, put it in, draw other stuff on it like maybe text.

By default, all window graphics are taken from the windowskin and they are always square(even though some windowskins are made so it doesn't look that way). But with some work you can make it look round or whatever shape you want, but it takes quite some work with margins and those kind of things.

I looked at the windowskin and saw the big box on the left.
Would it work if I drew a circular window inside the square, and then just make the rest of the square transparent, and then just figured out how to place the text?

If you wanted the round off the edges you'd have to change the border, which is the top right part of the window skin. The big part on the left is just the center part which is stretched to fit your window size and then the border graphic is drawn on top of it. The problem with windows in RGSS is that allot of it is created internally so they can't be edited as easily, and you would have to write your own methods to get them to display differently, it's not as easy as changing a few numbers around. Although there are other message systems where you can change positions around and stuff, those scripts usually totally rewrite the message window to get things to display how they want them to.