I don't think "collecting" (read; stealing) eggs and placing them on high mountains to be hatched and ridden by you (a random person) would go over so well with Manwe's servants..... I think having them be their own faction that requires great alignment to ride might be better.
Summoning other eagles to carry your troops will probably cause ridiculous amounts of lag, and wouldn't make sense id you've got 1000+ troops. You'd need at least 500 eagles for that.
I like the idea of an Eagle horn (mostly because I think its inspired by War in the North), but I think it should be a gift from an "Eagle Windlord" after you've reached a certain amount of Eagle Alignment.
Anyway, eagles are going to be added eventually, but its quite far away still.