The best time for the formation of a new realm was when Erik had all the conquered regions under his power in the duchy of Avamar, before the formation of two new duchies (one in Oligarch and one in Krimml). However, Erik is too faithful to Sirion to consider this option, even when he still have imperial desires.
I still think that the new duchies will become a new realm in a natural way, what will not change anything in Sirion, since we are used with the last borders. I doubt I will see a rebellion in Sirion or the realm falling apart because of internal problems. We have some key characters here to keep the things going on, like Ecthelion and Erik -- even when they have different ways to think how is better to lead our policies.
I don't like to see everyone against Sirion or everyone against Perdan, but the two realms have to face each other, since one have hegemony at the south and one at the north.
This is how I think I see what your describing will come to pass eventually.
"One day, one day hopefully sooner than later. The Sirionites will pour forth from over the river and strike out upon the lands of East island once again to put forth their strength south and Perdan will meet them head on with their banners catching the rays of the sun. The day that they meet will make the rivers run red with blood of Humans, Elves and possible some old Orcs if any be left lingering. The struggle between North and South, a war as old as the contient."
