You mean, despite Cass being only in 78th position in line to the throne and Ferdinand being Markus' heir? I'm very curious about how they would gain the crown, would you mind explaining how that would happen (also yeah, Nasir!)?
Venatori plotline trying to create instability in Nevarra by having a Civil War that will weaken the nation and have it ripe for Tevinter conquest, leading to a lot of deaths in the ruling family -- as Cass says, they're mostly very fat and lazy -- or other Pentaghasts conspiring with them. Civil War does happen despite Inquisition's best efforts. Also, political intrigue and blackmail and other things at getting other Pentaghasts to back Cass.
Cass is initially very much against the idea of her taking the throne, but warms up to it only because (as I said in another thread ages ago, probably my own) she does believe her duty to the Maker can be done elsewhere if not in the Seekers.
Probably gonna have to adjust it. Originally had it as post-game plotline but I'm thinking it should be during the main campaign.