The most popular theory I've seen, and one I subscribe to, is that Moonpaw is going to be the daughter of Nightheart and Sunbeam. I've also seen the same people or others point out that Sunbeam mentioned she wanted to wait a while to have kits (which is great !! i love family planning !!!). If she genuinely changes her mind or, worse, Nightheart changes it for her, that would suck. BUT ! I have a different idea !! What if... she just gets knocked up ? Unintentionally ? An unwanted pregnancy conflict that isn't technically illegal (like Bluestar, Yellowfang, Leafpool etc...) but just undesirable would be fascinating, especially if we're viewing it from the perspective of the unwanted child.
They almost certainly are not going to do that but would be cool !