Engine launchers may know immediately when an engine has died. These could send notification events to the Hub to begin unregistration immediately, rather than waiting for missed hearbeats, which can take a long time in cases where the hearbeat period is long due to large engine counts.

Heartbeats themselves could be entirely optional, but I think that's not a good plan, yet, as signalling from the Launcher assumes the Launcher is always running and all engines are started by Launchers, which is not generally true.