In antiquity, Teyrns arose from among the Banns, having grown powerful enough to move lesser Banns to swear fealty to them. There used to be many more teyrnirs before the rise of Calenhad, but he downsized them during his war for the throne.[1] In modern Ferelden there are only two teyrnirs; Gwaren in the south and Highever in the north. Teyrns are still owed fealty by vassals of both Arl and Bann ranks, whom they may call upon in the event of war or disaster. However they are also responsible for lending aid to them when needed. In modern times a king or a queen has the right to name a teyrn.
Fergus have nothing exept the title even if he get this title he would be a puppet to Eamon and as teyrn he does not have any right and opportunities to become a king himself and he does not have childrens..i am not talking that fergus is weak himself to fight for becoming a king so as other Couslands he would be a loyal dog for the Ferelden throne
The King or Queen has the right to name a New teyrn, as with the case of Logain, but no Monarch will be as stupid as to just take the ancestral lands from the oldest and one of the most powerful family in the Country and give to their dog walker. Especially in a country like Fereldan, where the Monarch does not have the absolute authority.
And why would a Teyrn be a puppet to an Arl, even if it is the most powerful Arl, It is has been stated that the Couslands are one of the most respected family, and if there is a living scion of that family, he will most certainly receive all the lands and titles that are rightfully his.. Fergus may not have any children right now, but at least he is not sterile, unlike our dear old Alistair, Fergus will become the teyrn regardless of your Warden' origin, the Witch Hunt DLC proves that.
http://dragonage.wik...he_Mabari_Hound
However you are right about one thing though, He would most likely be a Theirin Royalist just like his father, And would not even consider becoming King as long as there is a living member of the Calenhad bloodline.
However, consider this, If you made Alistair the King, then he will most likely be unable to produce an heir, and will have to answer his calling in 15-30 years, at that time Arl Eamon will be surely dead, and Fergus would have remarried and would have probably produced an heir. At that time he would have become the most powerful noble in the Country, So, who will fill the role of the King when Alistair answers his calling? Fergus!
Well, that's how it goes in my headcanon, anyway.