I realized something the politics of Avalon seemed to be missing - a claimant for the Vaticine to support as the rightful heir to the throne.

I was thinking this character could by the lynchpin of an exploration of dynastic marriage politics (at the moment that's the main thing I've thought out, I've left her personality undefined as I thought a player might be interested in actually taking on the role themselves and fleshing her out if they wanted to deal with all the baggage her situation brings).

So Richard IV (the setting's Not-Henry-VIII, father to Margharet and Elaine) had a sister, who wed a Montaigne nobleman before Richard's falling out with the Church.  They had a son.  During Margharet's Vaticine reign, as it became clearer that she wasn't going to bare children, that man was increasingly looked to as her likely successor.  He had a daughter.  Against the backdrop of the War of the Cross, to secure Vaticine unity (being undermined by King Leon's own support for the Objectionists in Eisen to weaken Imperator Reifenstahl), this heir to the throne of Avalon makes a marriage pact with the king of Castille, engaging our Not-Mary-Stuart to the future Good King Sandoval.

This doesn't go over well in the court of the Sun King, and our male heir ends up predecessing Iron Margharet.  Not-Mary-Stuart is taken quickly into the royal household of Montaigne for her protection (much to the frustration of Margharet and the King of Castille).

A wrench is throne into all Vaticine succession plans when Margharet dies, and Elaine reappears after being thought dead for years, in possession of the Graal.  Of course the Vaticine faithful don't consider Elaine legitimate, but the uniting symbol and power of the Graal made it impossible at the time to stop her from taking the throne.

10 years later, and Not-Mary-Stuart is a teenager reaching her majority.  Rumors circulate the Sun Court that if Morella Giancinni does not bare a son soon, she will be cast aside and l'Empereur (as Leon has since proclaimed himself) will wed Not-Mary-Stuart himself, crown her the true Vaticine Queen of Avalon making him King-Consort (a move that would effectively be an act of war against Elaine, who Leon is still smarting rebuffed all his overtures for marriage).

Meanwhile Castille considers the original marriage contract engaging Not-Mary-Stuart to Good King Sandoval to be still binding (he's a few years younger than her, but way closer than Leon).

In Avalon, Elaine has a soft spot for the girl - she herself was engaged against her will and effectively a prisoner to secure her half-sister's rule before she fled to Bryn Brasil.  But she also has no illusions about the threat a Vaticine claimant on her throne poses as long as she draws breath.  Some members of her court think killing the girl would be the most humane move to avoid a war.  Others - with awareness of Elaine's own lack of heirs - think that "rescuing" her to engage to a Graal-loyalist and hopefully produce heirs who can be raised in the Church of Avalon is the best move (James McDuff's name has been floated for this end, potentially uniting the thrones of Avalon and the Highlands completely).