You answered it yourself. It lacks efficient multicast.
What dbus really needs is an efficient kernel-level capability-aware multicast IPC mechanism, and a total rewrite of the user-space daemon to not be rubbish.
All the user-space daemon needs to do is provide a registration point for other processes to find and connect to each other via (existing) unicast or (new) multicast mechanisms.
My initial thoughts were that kdbus /was/ that IPC mechanism, but it sounds like that might not actually be true.
What dbus really needs is an efficient kernel-level capability-aware multicast IPC mechanism, and a total rewrite of the user-space daemon to not be rubbish.
All the user-space daemon needs to do is provide a registration point for other processes to find and connect to each other via (existing) unicast or (new) multicast mechanisms.
My initial thoughts were that kdbus /was/ that IPC mechanism, but it sounds like that might not actually be true.