At this point, this one gamesetup option is planned for the regicide victory condition.
The trac ticket was originally created with the goal of introducing a "regicide" victory condition of some kind, and it has a lengthy history; once this option is implemented, it should be closed.
This patch uses a dynamic template in order to prevent heroes from garrisoning, and so it requires the patch at #2951 and cannot be implemented until dynamic templates are added.
One of the largest issues with this patch is handling cases when players start with all of their units garrisoned on a ship, and (currently) this is only an issue with the skirmish map "Sicilia Nomad". In this patch, this is handled by spawning the hero using the closest possible entity on land, instead of spawning the hero on the ship.
Light AI support is included.
I haven't added component tests yet, as some things will likely need to be changed.