The only necessary hypothesis would be that the effects of his design are observable. It's otherwise an interesting question, but it's not particularly relevant to the (hypothesized) fact that something occurred as a result of design.
If one hypothesizes that "this is designed," one would expect to test (in some way) the observable traits by which we could distinguish between design and non-design. The identity and nature of the designer are not germane to that question -- though they would obviously become very important questions if a "design" result ever came up positive.
No one denies that living things are designed. Evolution says natural selection is the designer. This is a testable hypothesis.
What hypothesis does ID propose, and what are its characteristics. You are the one who said the characteristics of the designer could be known.