I think "designer" is largely in the eye of the beholder, but to me it would be like good, quality Marans, Olive Eggers, Penedesencas...all the dark egg breeds really...and any of the truly-blue egg layers.
Hearing "designer breed" immediately makes me think "overpriced mutt" whether it has to do with dogs, chickens, rabbits, horses, etc.
I understand people taking the time and energy to do a bit of crossing with other breeds in order to improve certain traits in their target breed, but there is no predictability with the genes. I mean, certain known breeds have their 'recipes' for creation known, like the chantecler.
Oh... ok! So, basically, I could say that I mixed Buff Orpingtons and Golden Comets (GSL) to improve the laying in the Orps (without the egglaying issues that come with the GSL's), and to get a bird with half decent broodiness, and call it a "Designer Breed"? I really did that, just so everybody knows... The one chick that came from them is a amazingly sweet, but surprisingly small girl. I like to call her my Orpet...
I have some more in the incubator (I hatched that chick under a first-time broody)... I may keep a few, but most will be sold.