I love the panel you are using and what you have done so far looks really good, I think some bear paw blocks , log cabin, or HST, or flying geese, there are endless choices in block you can add. good luck, and make sure you post the finished quilt, I love to see what people do with panels as there are so many beautiful ones out there.
Also you could do a search here and you will come up with a few panel quilts that are beautiful, also do a search on here and put in horse quilt there are some really beautiful ones there..