You retards. Aside the fact that is easy to just let somebody impregnate her than a contrived plot to give the power of the realm to an orphan you are forgeting something.
The fucking coordinate power.
As far as we know Eren discovered that if he has direct contact with somebody of royal blood he can use the coordinate (it happen when he punched the titanized ex wife of his dad).
And when he hold hands with Historia he can get high and have nice flashbacks.
Is almost 99.9% garantee that if he impregnate her with his seed then Historia will be able to use the coordinate herself, or at least her child will be able too.
There was no point at all on having Eren beign able to use the coordinate under direct physical contact with Historia if this is not going toward the route of her beign involved or having an heir able to use the coordinate.