To clarify some things - as discussion started by my post went in some strange direction that I didn't expect.
My first thoughts: Lumia could be capable of taking something from both parents and then conceiving a child from that (I'm fully aware what gods are). Even if parents are biologically completely unable to do that (like male PC and Ryn pair). Treating simple infertility is at level of her priestesses. This would be God intervention level of magic. Storking random child - simply an adoption - would be quite lame (IMO) for Goddess of Fertility miracle. Later child would either grow up in magical womb (that was that bonus bit), in surrogate or start of as a newborn - If that the case we might see when/if writes write something about it.
The why: I saw plans for Ryn heirs - and that is exactly why I wrote in this thread - unless it will mandatory part of quest I'm not interested in that content and I don't plan to read it. I always thought that this thread is in some way a wish list. Above is simply my wish

and I'm also aware how improbable it is
I should be more descriptive from the get go.