If you are thinking of adding a dragon of the south, that would not make sense. At least make it a dragon of the north, i have not heard of any dragons dwelling in the South. Smaug was in the North, and EA was smart enough to have Drogorth live in the North, as he should have.
For Isengard, when you get the fortress upgrade "crebain" or the crebain one, the fortress should get a special ability with the crows. One could be to devastate an aerial target, because it would be hard for a fellbeast to stop a bunch of crows from pecking at it
. It could do less damage to eagles because Eagles are birds (at least more than fellbeasts) and they are sort of the 'lord(s)" of birds.
For men, they should make Gwahier because Gwahier greatly respected Gandalf. he could summon up to 4 eagles to himself, which are permanent. As well, he could not summon more, until those 4 eagles are destroyed. Their should be a limit to summoning two each time, and once their are 4, the timer won't recharge until their are 2 eagles left. The timer for summoning power should not be long.
For the dwarves, The Eagle Lord (I know who you are talking about, The one from the Hobbit) could be summoned with 4 eagles, all permanent. Once the eagle lord is dead, he could be recruited as normal hero. He could have the same ability as Gwahier.
The reason I chose them to be able to summon that many is because when Gwahier and the eagle lord arrived with eagle reinforcements, they usually arrived with a lot of eagles, hence the "summoning."