Well I saw "theory" and got really excited. Luckily, I actually do have a Key-related theory. This may or may not be confirmed of debunked in the Little Busters! visual novel or in Little Busters! EX, but I've neither read the VN nor watched EX. If it was addressed in either, please don't tell me: I'd like to find out on my own.
I have a feeling that Shou Saigusa, the man who was introduced as Haruka's father may in fact be Kanata's father. At first, I thought of that being an ironic twist of fate the Futaki family agreed on the wrong child for being their heir, but when I rewatched the anime, I started to honestly consider it as a legitimate possibility. This is supported by the fact that she did visit him while he was in jail, this being an opportunity for him to tell her the truth if that was the case. In fact, I think he did tell Kanata that he was her father because every time she refers to him in conversation, it's always as "Dad" or other similar title.