I've been thinking this for awhile and I dunno if someone already suggested it.

What if our kings also had fertility levels too.

Like if the king is VLF and a lioness is VLF there'd be a higher chance of mutation or a king can be 'god' fertility that can balance out a VLf or vice versa.

Another idea too is that impression can also raise the fertility of kings too. Really just a loose idea that's been floating around