Innovations...

[Ferret]

It could be like in BI with the Western Roman Empire Rebels etc. only with set provinces,
e.g. If English held North America rebelled it could become the USA or something along those lines.

[katank]

Somethings to take from Rise of Nations perhaps may be the concept of dynamic borders. Being able to borderpush the enemy would bring increasing lands uner your control without open war and you being able to sit next to their cities while technically in your lands. Forts can also be made more useful by exerting border influence.

Some concept of supply lines could also be interesting and encourage expanding in an organized fashion, making it more risky to leapfrog hardpoints etc.