I think had we known more about Baelor's canonical wife and their relationship and marriage, or how they felt about each other, people would've written less OCs and more about them than they are currently doing, which isn't much. But we don't know anything about her as a character or if she's even still alive or not when Baelor dies. The only mention we have about Jena Dondarrion in the books is just her name in the Targaryen family tree which you can find in the TWOIAF appendix. If not for that, we wouldn't have even known her name.
Sure we don't know much about Jena, but we can grasp enough about her and their dinamic simply by looking at where they come from and the political climate.
Jena is a Dondarrion, a marcher house from the stormlands. They have been the number 1 line of defense against Dorne for thousands of years. Baelor is the first half-dornish Targaryen after one of the deadliest conflicts in westerosi history, and what's worse is that he looks more Martell than Targaryen. I imagine there might be some bad blood there at first.
We know Baelor was good at diplomacy, and that the marcher lords as a whole think that the dornish people are liars, so Jena might have hated this part of his husband's personality and thought him dishonest.
The marcher lords at the time were the ones who were more vocal against dornish influence and peace with Dorne, and they distrusted both Daeron II and Baelor. A marriage between them would have been brokered to keep the peace first and foremost, and both would have felt the presure of the very balance of the seven kingdoms resting on their marriage succeeding.
And we do know that it suceeded, because in the First Blackfyre Rebellion the only marcher houses that sided with Daemon were from the Reach and not the Stormlands, where the Dondarrions hail from. What's more, in the Battle of the Redgrass Field, Baelor personally led the forces of Dorne and the stormlands.
We can infer more of Jena's character from the culture of the marches: they have a strong martial tradition, they are no-nonsense people, and they have their own particular tradition of epic poems.
We don't meet Jena, but we meet Ser Manfred Dondarrion, who must have been either her brother or her nephew. We know he's callous, stuck-up and selfish. In the show we see he's necrophilic and, after seeing his obsession with people who share his hair color, probably incestuous. A difficult character certainly. From imagining how a woman might feel living in the same castle as he, we can imagine much of what Jena must have endured.
It seems to me that there is enough meat there to have an interesting love story without needing to pull an OC niece out of the hat. Also, Baelor already has nieces. If they really want to go the incest route and shipping him with Maekar is not enough, there is Aelora to consider.
