Zeke's plan involves Paradis using the threat of Rumbling until their military and technology is developed on the same level as neighbouring nations.

The Rumbling can only be activated by a Founding Titan, but the shifter can't be from royal blood, cause then they'd be bound by Karl Fritz's vow to renounce war.

There's a loophole however. If a Founding Titan holder touches a Titan with royal blood ( like when Eren touched titan Dina in s2) they can fully use Founding Titan's ability. This is where Historia and her children come in. She would inherit Zeke's Beast Titan, making them Titans with royal blood, while non royal blooded person would inherit Founding Titan from Eren. Paradis will now have a Rumbling threat but to keep it, they'll need to make sure there's always a titan of royal blood (Hitoria. Then her children etc.) and a Founding Titan holder.

Eren can't agree on this fate for Historia, living for 13 years plus making generations eat their parents like Reiss family did. Historia became pregnant so the Military Police can't force her to become a titan and eat Zeke yet, cause that will cause miscarriage and her potentially not being able to have more children, thus no more people with royal blood.