Just think of it as making a Round Robin quilt and just keep adding borders until you have it the right size. If you need it rectangular, add extra rows at the top and bottom. Rows could just be pinwheels or star blocks. Borders could be pieced like flying geese or piano keys. Appliqued vines with flowers is nice.
Quilters Cache has a lot of nice border patterns to look at:
http://www.quilterscache.com/B/Borders.html