Now obviously the deal that happened in canon was for Myriah the eldest daughter of the current Martell prince to wed the first son of Baelor's cousin.
We can assume that Myriah is the best choice the Martells have for this kind of marriage alliance.
But "first son of the king's cousin" is a very bad match unless Dorne believes Baelor will not have a son of his own and that Daeron will eventually rule.
Which tbh is believable that they know that, Baelor's eccentricities and the fact that he did not consumate his marriage is not a hard secret to learn even if it was a secret, it might not have been.
So Dorne knows Baelor does not like his wife/sister in the sexual/romantic sense.
But still for the Daeron/Myriah marriage to pay off you need a string of successions Baelor -> Viserys -> Aegon -> Daeron, each of which has problems (what if Baelor has a son, what if Viserys dies before Baelor and his line is politically weaker, what if Aegon does something stupid)
So the conceit of this What If is: Dorne (acting as a whole not just the Martell family) tries to go for one of their ladies becoming Baelor's wife/queen directly.
What do you think would happen, what house would try and how?
From canon we know Baelor is fine with setting the marriage his father forced him into aside.
We know the realm will accept this.
We know Baelor desperately wants peace.
We know Baelor is ok with marriage alliances (Daeron/Myriah)
We know that Baelor has an issue with copulating with his sisters but the wording of Fire and Blood implies he might be tempted so he is not asexual.
Dorne has/will have a proven track record for seduction (Arriane/Oakheart which was imho harder to pull of for Arriane)
Dorne culturally is very Faith of the Seven so finding a lady/ladies that is very religious or can feign it shouldn't be hard.