I may have missed something here, but from where I'm sitting Ferelden will indeed be in need of a new monarch pretty damn soon. Alistair and the warden are unlikely to sire children, and Anora doesn't remarry if she rules alone.
People have suggested the mages could help wardens overcome their infertility. But that's speculation; we have no reason to think this is true. The Dark Ritual proves their infertility can be overcome by magic, but we don't know if the child the ritual produces is a normal, healthy, human (and non-magical) child, and we have no reason to think anyone other than Morrigan (and, almost certainly, Flemeth) knows it.
However, I'm no lore geek, but doesn't Ferelden already have rules in place for this sort of thing? Isn't the Ferelden monarch at least party elected? Like the Anglo-Saxon kings, where being the son of the last king merely made you lead candidate, rather than automatic successor? Or have I got that wrong? Because a country prepared for such eventualities might well avert civil war by resolving the issue through diplomacy and politics.
And even if Ferelden's monarch is dictated entirely by bloodright (which would be odd since the Landsmeet accepts either a commoner queen and/or a bastard king), isn't there a closely-related cousin somewhere? This would be a more interesting scenario, imho, if Anora ruled alone, because then the question would be whether they should be looking for a Therin cousin or a Mac Tir cousin.
Also, Fergus seems to be of an age with Anora. So if she dies of old age, it is likely Fergus will be either dead or a very old man himself. I think the champions of the Cousland bloodline should be pinning their hopes on any (further) children Fergus has rather than on Fergus himself.