Responses
It's been established that Historia needs to be an heir, both because she's the queen and because her family if the original holder of the Founder Titan, and so has a bond with it. With Eren being the new holder of the Founding Titan (and the War Hammer Titan now too), if he father's a child with Historia, it'll be that much stronger. So if Historia has to have a child with someone, it might as well be with someone who will produce an extra special/strong offspring. At least that's the theory I heard.
Can someone explain, how did you guys come up with Eren's the father of Historia's? I mean he didn't said its his? Im confused ╥﹏╥