Not sure if this would be possible as a plugin, but …
When I set up a task as WAITING, I often include a sub-block with a block reference to the task that has to be completed first. It would be cool to have a plugin that scans for tasks that have workflow status WAITING and have a block reference to another task, and then scan that referenced task for DONE. If the referenced task is DONE, the WAITING task switches to NOW (or DOING, or whatever the user specifies in plugin settings) automatically.