Sherrie, you obviously can do diagonals in addition to horizontal and vertical. You probably could do a diamond design if you started with a glued up panel that was big enough to span two studs. You could work the design out from there cutting 45 degree miters at the 4 corners of the panel.
Do keep in mind that the more complicated you make the design, the more time and money you will spend. A complicated design with a lot of cuts usually means that you have a lot more wasted stock also. You would have to face nail and fill the holes on half of the wall. Furring strip nailers might be necessary for complicated designs also.
You should also be able to put in a band or bands around the room with contrasting wood. That might add a lot of interest without too much cost.
Hope that your miter saw is in good shape and good luck. Henry in MI