Welcome Kali. Of course this has been discussed, but I'm always up for this discussion... mayhaps I'll learn or re-learn something of importance.
There have been a couple threads like this, but
The Dragon Has Three Heads is by far the most popular and thorough.
I'm not sending you off there to post... I just wanted you to know where else to look for other possible ideas...
Now on to your post...
Yes, Jon seems the most popular and most logical choice to join Daenerys as a head of the dragon. And if Dany is incapable of bearing a child, then the third head should be a female who can bear Jon's child. I don't think that the lords of Westeros would take seriously the idea of an adopted child by Dany.
Who are the likely candidates?
Sansa. She has red hair... Jon likes red hair. She's the daughter of Eddard Stark. She's making the switch from pawn to player in the great game. She's the eldest child of Edmure's sisters... if Ed dies without issue, then either his uncle, nephew, or niece would inherit Riverrun. Also as cousin to Robert, Sansa figures into the Arryn line of succession as well. So Sansa actually figures into the lines of succession to three of the six great houses. One obstacle is that Eddard was one (if not the primary) of the supporters of Robert Baratheon in his overthrow of Dany's father.
Margaery. She's already been married to three kings. Why not a fourth? Renly thought she looks like Lyanna... Lyanna is thought to be Jon's mother... so why not completing the circle by Jon marrying his mother's doppleganger? House Tyrell can field the largest number of knights in the Seven Kingdoms, so Dany must reckon with them.
Arianne. She was secretly betrothed to Viserys for at least a decade... so why not just marry her off to Viserys' sister? It seems that Arianne is no stranger to menage a trois'. The Martells have been staunch supporters ever since their incorporation into the Targaryen kingdom.
Arya. Another Stark daughter. She's geographically closer to Dany than any of the other candidates. She and Jon have a great affinity towards each other... in fact, when Jon thinks of Ygritte, he often compares her to Arya. Eddard told her that she looks like Lyanna... another possibility to bring Jon together with his mother's doppleganger.
Jeyne Stark, nee Westerling. She may be carrying Robb's heir... if so, then she's got a strong claim to Winterfell.
Asha. The Ironborn will remain a pestilence upon Westeros until a strong ruler can defeat them at sea... this most likely means a united Westeros will be first needed to even attempt to deal with the Greyjoys. Marriage might be the key to re-incorporating the Iron Islands back into Targaryen hegemony... that's how the Targs finally pacified Dorne. A problem with a Dany, Jon, Asha triumvirate is that none of them are Andals nor hold to the Faith of the Seven.
Val. How numerous are the Wildlings? I don't know, but she could represent the largest incoporation of people's into Targaryen rule since Aegon I won on the Field of Fire. Again, I don't know that the Andals would stand for this. Princess is a title not lightly ignored.
Myrcella. She's a longshot for sure. House Lannister probably will be exterminated for Dany to rule comfortably... especially the males. But Dany has a soft spot for the defenseless... and Tyrion (who we assume is going to join Dany soon) loves her too. She gives Dany her connection to the Andals she needs.
There are thousands and thousands of noble born ladies and girls to choose from in Westeros, but these are the players from the major houses and power bases.
I must also say that I'm not sold on Jon being a head of the Dragon. I think there are other unrevealed Targaryen bastards out there still... and Tyrion is my favorite candidate. The idea of Tyraen Targaryen has not received much support, but I like it. Even crazier is Jaeme Targaryen... hey it explains his penchant for incest.
Nor am I sold that Dany is barren. Mirri lied to her about a number of things. If this is the case, then a number of men from the leading houses become eligible to be heads also...
Loras. He'd need to be forgiven for serving Joffrey and Tommen, but hey Robert forgave Jaime and Barristan.
Quentyn. He's already on his way to woo Dany.
Victarion. Ditto.
Trystayne Martell.
Harrold Hardyng. His tenuous connections to a title and land, plus his name, seem reminiscent of Harald Hardrada... of English and Norse history.
Bran. He is the real heir to Winterfell. He seems destined to fly. Tyrion told him that on horseback that he'd be as tall as anyone... well, how about on dragonback? His inability to father children would never complicate the line of succession.
Theon. I don't know that he'll even retain any sanity by the end of ADWD.
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