Also yea, this issue has been nagging at me for several years now. I am one of those people who like to see Royal Dynasties continue, regardless if they are good or bad for the country at large. So seeing that Alistair still has no heir troubles me greatly. The Theirin Blood unites Ferelden in ways nothing else can. They take so much pride in the legacy of Calenhad that it is not something I can see being so easily replaced. The royal blood is part of their national identity.
Just as Loghain Mac Tir was given nobility status and became very popular and highly respected following River Dane, the Hero of Ferelden was elevated - both in actuality, becoming the Warden-Commander and Arl of Amaranthine, and also in the hearts of all Fereldens.
It was an ideal time for the HoF to coalesce power to ensure the long-term stability of Ferelden - especially a Cousland HoF.
As for Celene, considering The Masked Empire, I doubt she will ever lay with a man to begin with, nevermind long enough to get an heir established.
It takes only a few minutes to conceive a child. If Celene deems it important to birth an heir, I would expect her to make that sacrifice.
The whole heir to Ferelden situation was a HUGE consideration on my part when going through the Landsmeet. Anora's alleged barren state meant that I wasn't about to make her Queen in any way.
It was also a huge concern of mine, though I didn't take rumors about Anora's infertility too seriously.
Ferelden would risk some sort of war of dynastic succession at her death, whether she ruled alone or with Alistair. Even though politically it would have been easier to marry her to Alistair and have done with it. I chose to give Alistair alone the crown because there was a good shot, if still not the best, that he'd be able to father a child with another woman.
I put Anora on the throne and sent Fergus Cousland a-courting. Even if Anora turned out to be infertile, Fergus could have produced bastards to keep Ferelden's succession secure. Fergus had already fathered a child. Uniting Ferelden under the leadership of the 2 most powerful noble lines seemed to me to be the best way to ensure ongoing stability.
I understand that BioWare needs to try to consolidate the original choices as the timeline goes on, but I do wish there was some acknowledgement that if Alistair reigns alone that he married. They could have still written in that he's been unsuccessful in producing an heir, likely due to the taint, and have the same end result of the line of Theirrin kings dying off and a new dynasty establishing itself.
Here's the thing: We've seen that Bioware intends to respect player's choices regarding Alistair's fate. He can show up as a King, a Grey Warden, or a drunk in Kirkwall, and also can appear in DAI. He may also have been the one who made the ultimate sacrifice in DAO, or he may have been executed. They would have to retcon all of that in order to set Alistair as canon King.
If the OGB exists, it could be the HoF's, Alistair's, or Loghain's.
That events were set up to enable Fergus to survive DAO (sans family) tells me there is a pretty good chance that, if the future rulership of Ferelden is ever addressed, Fergus (or his heirs) may play a part in it. Fergus' survival, fertility, and lack of family are the only factors that are consistent through all playthroughs.