No one will ever know there was to have been appliquéd Dresdens in the corners. Whatever design you chose to insert in the corners will look like it was planned that way. After seeing what you've already done with this quilt, I wouldn't begin to advise you - the designs you have already done tell me you'll pick the right ones for the rest of it!
Also...thank you for telling us you marked that whole grid first. That gave me a "whew" (so that's how it's done) moment! Love your quilting.