For me - my dream dahlia would have the inky black foliage of a Bishop dahlia, but finer cut- almost lacy in effect. The colours would be the pale milky colours of ice cream - peach, strawberry,lemon and vanilla. The dahlias would be loosely and gracefully bushy but with long stems and dangling nodding single flowers - almost like a turks cap lily in effect or a throwback to a wild dahlia. The grower at Queens Valley Farm has something like this dahlia in my mind (without the black foliage) with the dahlias she hybridised 
This photo is taken from her website - which is very interesting as she restored a historic property which still had historic dahlias growing on it saved thru the decades
https://www.queenvalleyfarm.com/