What different components would a daemon expose via the message bus, respectively knowledge store?
For one, methods for remote control. Since RPCs aren't very flexible, the communication style should use message passing. Waiting for response needs to be both aware of possible timeouts and communication failures.
Passed objects can't be too complex, since every interacting language/environment needs to be able to access, create, possibly manipulate them, as well as to minimize the amount of overhead during serialisation. At the same time the schema of messages isn't fixed, so there is an obvious problem for very strict type systems, in that either a fixed-schema wrapper can be used, which would need to be updated at some point, or be kept backwards-compatible, or a very dynamic representation with runtime checks would have to be implemented.
Comparable string based messages from e.g. Plan9 are too simple on the one hand, whereas a protocol like DBus might be overengineered(?).
An important point to consider is the in(tro)spection features of the
global system. It should be absolutely possible to easily see and edit
messages as well as stored data in either a text-based or convertible
format. Also, commands and objects should have runtime documentation in
the system, so that a hypothetical call like
describe object would
display text- (in the terminal), or hypertext based documentation (in
Naming clashes have to be solved by a one or multi level package system. Since the naming is both shared and global, typical approaches include reversed domain-names, UUIDs and prefixing of identifiers.
Session management would include saving and reloading of state, even with different session names. A separate session manager would take care of initialising applications based on the saved state.
For both text- and graphical UIs methods to switch to the active screen/window need to be provided. Copy & paste functionality might still be done via the window system, or additionally via messaging, which would allow connected system, as well as text and graphical applications to exchange content without any problems.