I can see Kidd being Luffy's WB. Rogers sought to conquer the seas, right? Luffy's definition of pirate king is having the most freedom in the seas, if I recall. Already in a way, both are different. Luffy's WB doesn't exactly have to be like Whitebeard, but be Luffy's rival. They don't necessarily have to be on good terms, and I can't see a Shichibukai being a WB. Kidd seeks to conquer the ocean, as opposed to Whitebeard seeking family. Luffy seeks to have the most freedom and find One Piece, while Rogers sought to conquer the sea and travel to the end.
Blackbeard is more like the villain, considering he caused Ace's death and set off a new age that threatens so many lives. His power is the only thing that can make him Luffy's WB, but his strength is what makes him Luffy's enemy more than anything else. I can't see Coby being strong enough to be Luffy's rival either unless he gets a strong Devil Fruit and trains hard.