One of the problem with secession is the complete cut it makes with other realms. Even with religious or guild message groups, people don't go back and forth between realms.
If you look at the middle ages, drafting people who lived in other realms but had the right connections/bloodlines/ideas to become leader of a realm was almost the norm rather than the exception. I would really like to see, for example, the ruler of a vassal "realm" resigning because he has been offered the position of Duke of the capital of the mother "realm", or the general of a frontier realm getting called back home to become marshal of the Imperial Guard, or the Imperial Judge being delegated to bring back order into a frontier march.
Not only do I never see that in federation of small realms, I am pretty sure a lot of these transfers are actually forbidden by game mechanics.
If there were decentralized bigger realms, rather separated small realms, this is the kind of thing that I would like to see.
(And at some point pigs will fly, I know. It's nice to dream.)
