The new rules feature is great. However, the first custom rule I imagined, and wanted to create, would involve using a tag as a trigger or as an action. An example trigger would be “Run this rule when this particular tag is set on a task.” An example action would be “Set this particular tag on a task whenever this rule is triggered.”
On a technical note: Infinite cycles (loops) could possibly be created from such triggers and actions, but preventing long cycle chains would be a consideration with pretty much any types of automated rules.