I'm going to go with the 200mm f/2.8L II USM prime lens, unless it acquires image stabilization, in which case it would become the 200mm f/2.8L IS USM.
The reason is because it's soft and has very low contrast wide open. Being an L-series prime, it should have better image quality than the 70-200mm f/2.8L IS II USM, but it doesn't.
You must have a bad copy. I admit I've never used one but I can't recall anyone describing that lens as soft.
Anyway, I confidently predict there will never be a 200mm f/2.8 III, nor a 200mm f/2.8 IS.