I added some onion just because I had a surplus of them, and it worked out well. I would suggest cutting them small though, so there isn't sharp onion flavor. Might also be interesting to add carmelized (or at least sauteed) onion.
This is an excellent dish to bring for potlucks or BBQs. It is quickly becoming a favorite in my house and with friends and family. So easy to pull together too.
I made this for a backyard BBQ this past weekend. All the wives asked me for the recipe - I doubled the recipe and added more vinegar and sugar than it called for as we like it "sweet and sour" I also used just the celery leaves because in our family the kids don't like celery -