Adding another trigger type to RunBooks would enable support for the GitOps style of Kubernetes administration.
The type of trigger would be a source control trigger. (This is very common in autobuild systems.) Basically they come in two flavors: Polling and Listening. Polling just regularly asks the Source Control System if it has changes in the scope it is monitoring. Listening is more complex, but it involves a webhooks, sockets or some other way for the Source Control System to notify the trigger that a change has occurred. (Polling is easier and would work great for this feature suggestion.)
GitOps is a fairly big deal in the Kubernetes world. Adding the ability to have what is checked into my Operations Git repository reflected automatically in my Kubernetes clusters (via a trigger to run my runbooks) would be a win for Octopus Deploy.