Ferns, and personally I like epimedium, also in those conditions, seems like blooming plants like daylillies, Iris would be nice! I like your idea of moving the colorful yuccas to the back and keeping the others up front as bookends. One thing that always happens for me though is that some of the plants do better than others, especially if you're taking smaller "starts" so you may want to not have each side "balanced" symmetrically - that way it won't accidentally look off balance if anything changes
