I think you have some wonderful ideas already. I loved what the other ladies suggested. WHen I have a panel and I want to make a twin sized one, I just add strips in coordinating colors. But since you have other horse fabric and want to make a queen sized, I'd definitely get more creative...also because it is for your granddaughter and not a charity quilt. (I've done charity Spiderman and Batman quilts this way...just by adding borders.).
Since you have so much space on each side, the sky is the limit...really.