Code Geass Continuation?

Hi I'm writing a huge continuation fanfic that takes place directly after the end of R2. I'm stuck on making up a worldwide conflict that affects all of my character groups (Kallen and Suzaku in Japan, Nunnally and Schneizel in Britannia, C2 and Lelouch in hiding) so do you have any ideas for how tension could be created between the UFN and Britannia? Or maybe something to do with Geass?

I have an idea for how to continue my story but am stuck with something....:
"Empress Nunnally gives all nations a choice -- remain part of the Britannian
empire, be a commonwealth of the empire, or gain full independence. Under the
new order (they're not united like the UFN) peace and prosperity reign --
until national rivalries are renewed. Jealousy and corruption threaten to undo
the world Lelouch has created, and it is beyond the abilities of Nunnally and
Suzaku to save this world. As the world creeps closer to war, Kallen observes
from a distancewondering if there is anything that can be done. Until one day
she comes home and finds C.C. waiting for her." But Nunnaly understood in the
end what Lelouch wanted, and she also had an idea of what had to be done. She
wouldn't do this randomly. I need to give her a good reason, if I go with that idea at all. Thanks.