We're all fucked !
They didn't do the required sacrifice this year, they didn't BURN THE MAN and now the ancient plague has awakened once again. We're all doomed !
DOOMED !

Concept: the endlessly-resurrecting Ancient Evil's plans are thrown for a loop because the monarchy was abolished like fifty years before his latest resurrection. He can't even track down the last scion of the Goddess's Chosen Princess, because every possible form of magic or ritual he could use to his advantage requires an actual, y'know, princess princess, even if she's just exiled/lost/unknown. And this wasn't a power-hungry usurper kicking them out, or the kingdom being destroyed or sealed away by other means, or whatever; it was the kingdom itself voluntarily changing into a full Republic™ and specifically declaring that the members of the royal family are officially Not Royalty Anymore, so now she wouldn't count, if she even exists.

Some options:

• There's nothing stopping the not-a-princess from becoming the hero instead!
• Destiny doesn't care how it comes about as long as the destined thing actually happens, and the Goddess could already see that she was going to have to adapt to the new circumstances. The role of "hero" has always gone to whoever had the courage to bear it; why not the role of "princess"?
• The Ancient Evil decides that the best way to perform the rituals he wants is that first, he needs to get the country to reinstate the monarchy. It shouldn't be too hard to get involved in the ex-kingdom's politics and find a patsy who's power-hungry, likes the idea of being royalty, and has a daughter, right?