@Puppy0 Actually, I'm piloting a few of the important houses but they're all mostly free. House Targaryen is GM controlled and House Tyrell and House Arryn are PC controlled but the majority of the Paramount Houses are open, just some of them have pre-established characters. But House Martell, House Tully, and House Greyjoy are mostly open-ended while the Starks, Lannisters, and Baratheons have some more development. Pretty much all of the important secondary houses have plenty of room to maneuver too.

@Puppy0 It'll be the Martells because it's always the Martells.

@Puppy0 Cool. Just keep in mind that Arianne is now the Princess of Dorne, Aegon Targaryen (Faegon) married one of the Martell Princesses (ambiguous whether Arianne or one of the Sand Snakes) and had children, and Prince Viserys married another Martell heir named Nymeria and fathered children by her. So the Martells are pretty well tied to House Targaryen.

@Puppy0 Yeah you can do that. Side note, is that Aegon's kids are also dragon riders, and we'd have to come up with a valid reason for Jon and Dany to let a Paramount house have control of their own dragons. Right now, the only other house allowed to have dragons is House Velaryon (who have always been close to the Targs and are the only other Westerosi house to have dragonriders historically). So Aegon could just be a Paramour of Arianne's as well. It depends on what you want to do.

I would advise having another character be the heir to Dorne, someone you designed to be the heir. If you consider the Sand Snakes and Doran's cousin(s?), House Martell is actually pretty big. Nymeria is just one of many Martell heirs. Viserys is the Prince of Summerhall and the heir to Dorne has to carry the Martell name, so it might cause some awkwardness if one of those titles doesn't go to an heir with the right name. Plus Nymeria isn't really heir material haha.

@Puppy0 Yeah, totally you can have a son be the heir. I alluded to there being a Martell fighter named Oberyn who's helping to suppress the rebels in the Red Mountains, and you can have him be the heir if you want.

Hmm, well for the Arianne situation, Aegon is always going to a dragon rider because he's the third rider from the Long Night (in this version anyway, who knows in the real story). Well, perhaps Aegon has a Paramour and his bastard children are dragon riders? Dorne is a bit looser about these things so Aegon can be married to Arianne but still have some sally on the side (damn you Sam!)

@Puppy0 No there's nothing known about any other Martells and we don't even know who Oberyn or Nymeria are the child of and what their place is in the succession, so all of that is entirely up to you.

Basically, you can still do pretty much anything you want and you can just regulate Oberyn and Nymeria into the corner if you want.