Unfortunately the boon you get at the ending of Origins isn't carried over to the sequels. My main warden was a Cousland, married Anora, and also chose the boon that grants him Gwaren. That puts Couslands in control of both teyrnirs in Ferelden, so in that case I'd imagine Fergus and his heirs would have the strongest claim on the throne and would be able to move Ferelden towards a more centralized monarchy.
Kieran, being the Warden's bastard, would be the only complication and could cause trouble later with his witch thief mother backing him. Leliana was his mistress too, and I nudged her towards being a ruthless Divine in Inquisition though I don't know how that'd play into things. Unfortunately, Inquisition was bugged and had both Morrigan and Leliana in a loving relationship with the Warden, even though he stabbed Morrigan in Witch Hunt.
All of this is too compelling and complicated for BioWare to design around, however, if the timeline ever reaches that point. What will most likely happen is Ferelden, with its love of democracy, will rally around Teagan in a Landsmeet and choose him unanimously. Thus, an ineffectual arl will rob the Cousland house of its rightful place and glory. 