Rather than dealing with the headaches of always giving an heir to our Ferelden monarchs, I see three potential solutions that the devs may go with.
1. They purposefully leave it unmentioned. Until it becomes relevant to the plot, why even bother mentioning an heir? Alistair and the Warden still have ~20 years of life left to go before their Calling. And Anora is a little older than them, but isn't a Warden, so she should have a little more time then them, most likely. So for at least 20 in-game years of Dragon Age, it's not a serious issue. This allowed headcanons to be relevant for as long as possible. But eventually, if Dragon Age continues into the 9:60-70s, it will be a plotline that the devs can no longer ignore. But that might be three or four games from now. That comes out to about 15 years, at the current rate they're making DA games. The fanbase should not have to be that patient to see this succession crisis, IMO. I know this solution simply delays the decision, but I see it as the most likely immediate outcome.
2. Anora always has a child, and this will always be the heir to the Kingdom. Anora is always alive. You can't kill her, unlike Alistair. Her fertility is not set in stone. I think the devs might decide that no matter what happens, she has a child. Be it the Warden's, Alistair's, some other consort. Let's not forget that Anora is still the Teyrna of Gwaren, regardless of whether or not she gets the throne. A worthy marriage-aged woman with plenty to offer, should a man meet her standards. And the devs have already said the Epilogues of DA:O were mere rumors and hardly canon in any sense of the word. So none of this 'Anora stays single if she rules alone' stuff. Anora understands the value of a good marriage for uniting a weakened country. I think it is a valid route for the devs to say that Anora always has a son/daughter to carry on the Mac-Tir name, and they are the heir to the throne regardless of who the Warden gave it to.
3. There are no heirs between any of the monarch pairs from DA:O, and some other figure has to step in. Whether Bioware decides for the player to take an active part in the succession crisis to follow would remain to be seen. For all we know by the time we get to that point in the Dragon Age, the action will be taking place in Tevinter. If this ends up happening, I think the only real option amongst the Ferelden nobles we know of are Teryn Cousland and Bann Teagan, with Teyrn Cousland the heavy favorite, I think.
Personally, I like option 2 the most. And not just because I like Anora. I think it simplifies things, but allows some level of consequence for player actions. This child could be of Cousland or Therin blood as well as Mac-Tir. But the core character could stay the same, since he/she would have the same mother. Also, it doesn't leave me feeling like my choice in DA:O didn't make any difference.