For now, the TargetList only supports:

• processes
• frames (in a limited way, for ex we don't support fission frame, nor support listening)
• workers (also in a limited way, for ex, we don't expose Service workers, nor frame workers when the top level target is the parent process target).

We should also add support for addons, so that the Omniscient Browser Toolbox can debug them as well.
A first step could be to add a LegacyImplementationAddons, like LegacyImplementationProcesses to the target-list module.
Or implement something like bug 1593938 straight away. But note that bug 1593938 should pave the way first as the actual API to implement in the actor side isn't 100% defined yet.