I mostly prefer the rounded corners on my decorative couch pillows... just don't like those corners to be empty and then get floppy if that makes sense. (zdesignathome.com)
Do not just stop at cushions, get pretty cushion covers to go with them — get multiple couch pillow cases to have them available readily. (myntra.com)
Toss a few fun checkered yellow pillows on the couch (lauralily.com)