I know that in shogun 2 once you become too powerful a "realm divide" event will occur where all of the clans unite to try and stop you. Will there be something like this in Sengoku?
What a ham-fisted bit of game design.
I think the idea of the realm divide is good and other sengoku-period games have something similar(notably Nobunaga's Ambition), but its implementation in Shogun 2 was a bit ham-fisted. Someone taking the lead should get ganged up on, but it should be a polarizing thing, with a few allies going to the leader and most of the rest forming a coalition against, rather than a deterioration of relations with everyone, including your vassals.
On the other hand, there should be allies for the leading faction as well. Historically, the Tokugawa clan did have quite a number of allies because they offered them the lands of the clans that were defeated. Generally speaking the AI should try to figure the best way to try to allow the AI clans to survive. There should be two main issues which are the size of other clans and their relations towards you. Smaller clans should generally be more friendly towards the leading clan and if their relations with you are good then it should be less likely that they would start a war against you. So, if you control the needed number of provinces to become shogun then there should be a theoretical possibility to win the game diplomatically.
I know that in shogun 2 once you become too powerful a "realm divide" event will occur where all of the clans unite to try and stop you. Will there be something like this in Sengoku?