The problem as i see it with continuing the pattern to add 12 inches per side is that you would not complete a whole pattern of the diamond.....so that might look strange on the "drop". I think i would border what is in the photo, then decide what i wanted to have showing in the bottom and side drops.
Are you going to do pillow shams, or will this go up over your pillows? That will also help you decide about the top and bottom. I usually do about 12 inches of solid fabric at the top --- why do fancy work that will be under the pillows?
I'll be looking forward to seeing what you decide - as i have really liked seeing your color choices and pattern on this one.