If they were to do relics and mythics justice in comparison to the difficulty of making empyreans, I would say mythic WS's should do about 6x as much as empyreans, and relics maybe 4x's. Or at least have better utility.

But I would accept mythics or relics being just a small upgrade, as long as they were the best.

Right now however nothing really seems to come close to empyreans, besides the infamous Ryunohige. Ragnarok might have a chance, but compared to Caladbolg, I doubt it.