I think I would go for borders. If you think about most borderless decks, the backs are usually some sort of very repetitive pattern, like the Bee's, Stickman deck, even the new D&D vintage decks. I would want the back to be very intricate, with curving branches, maybe leaves, even whole trees and roots. I think a border would work best with that. I've already told my sister and she's going to be designing it with me, patternwork is her speciality!