oooh pls tell us more about your reversal AU! Is Dunk still a bastard? Does he have a reputation for being a softy? Hows the strength of his house?
What makes him really fall in love with Baelor once they meet? Does Maekar make fun of Baelor for so flustering in front of his celebrity crush?
Nope, Dunk's the trueborn son of King Arlan's late nephew. So he's 100% legitimate and thus has a very, very solid claim on the throne.
As for how he became an orphan...it's a bloody story, I fear. The Blackfyre rebellions still took place in this AU. Not as a bastard Targaryen claiming the throne, but instead as a bastard Pennytree doing so. Arlan spent much of his youth dealing with them alongside his own children and their children. It was a horrible massacre both for the royal family and the common people that ravaged the lands.
It was a quiet night when it happened, really. Beautiful too. Mild in nature, with a clear sky and silent nature all around. Dunk had been barely one and ten at the time. He used to have a large family back still despite being the only child of his parents, with many cousins and aunts and uncles. No one truly expected him to ever sit upon the throne, being the king's grandnephew, and so he enjoyed many freedoms. He was training to be a knight, actually! A Kingsguard had been made his Ser a few years ago and the boy was actually dreaming of joining them himself one day. After the war was over and he could return to Kingslanding. King Arlan had brought most of the women and young children of his family to a faraway residence during the happenings of the rebellions in hopes of keeping them safe that way. Certainly not a dumb idea in and off itself....
.....but it did not save anyone. Not after the Blackfyres had gotten notice of where the king kept his family and sent their agents to take them out. It was a horrible bloodbath. Assassins snuck in under the cover of an attack, by the time anyone noticed what happened almost every member of the king's family, alongside Dunk's Ser had been killed. He himself only having survived in the nick of time due to his mother's quick wits, telling her son to hide.
Dunk was the only survivor and the only possible heir of King Arlan after it all was over. Some say the old man never fully recovered. The young prince was quickly taken into his Ser and thrown into the classes he'd need to prepare himself for his future role. Constantly guarded and watched over closely to make sure not a single bit of ill would befall him. The boy was never quite the same after, either.
The gods allowed him to live in place of the rest of his family. Dunk thought that must mean something, right? Surely he must make the sacrifise worth it. To honour the parents and cousins and aunts and uncles he'd lost. What right does he have otherwise to stand where they lay buried?
So he devotes himself to his people. It's a coping mechanism tbh. And in turn ends up making a reputation for himself both as a "soft-hearted fool not fit for the throne"..... as well as being a freaking messiah figure for the small folk. Seriously, Dunk is POPULAR with the people. If he was the champion of the people in canon, he's their god now. The smallfolk adore him. Ser Duncan the Tall, Prince Duncan of House Pennytree, The Good Prince, The Kind Prince, the Prince everyone cannot wait to begin his rule. He's known for taking in smallfolk into his service, for defending the innocent, for always remaining humble and kind and open. You best believe that this man is running at least one or two orphanages and hospitals for the people. Whenever there is a royal procession with him in it it takes ages 'cause Dunk takes the time to actually talk and interact with people. The lords hate his guts but also know that they CANNOT afford to mess with Dunk or house Pennytree by virtue of their popularity, a fact that, funnily enough, also strengthens Arlan despite his heir lowkey antagonising a good chunk of highborn officials.
It's kinda similar to Egg's rule. The Lords do not like his reforms a bit but won't rebell openly yet, partially because they know that their smallfolk will betray their asses as soon as Egg/Dunk ask them to.
The reason for why Dunk falls for Baelor is the same it is in canon btw. He sees an honourable knight who holds to his vows and oaths and his mouth starts watering.