Required interface of serialisation objects
Now, there is always a Serializable interface required to be serializable. This works, until there are more than one places in the app where things are stored.
In CampApp for example there is routing/session configuration of stateful & model/database configuration of stateful. It makes sense have these separated and easily spotted in code, for which use-case object has been intended to be used.
So if, there is a restricted mode, of Stateful, that would allow to serialise ONLY classes that implements some kind of (marker) interface, it would prevent you to accidentally serialising something, what was intended to be serialised elsewhere and then assuming that something is not used.
Currently this behaviour can be simulated using External serialises, which is very impractical.