Sounds great but how do you do that? The same system that mounts the filesystem with the key file in it (fstab) is going to try and mount the filesystem inside the LUKS container. This will fail since the container is not unlocked. You are supposed to be able to add disks containing key files in /etc/default/cryptdisks so they are mounted before the container is unlocked but I didn't have much luck with that.
We might be talking about two different things.
If you are thinking about auto-unlocking the root filesystem or any related FS's that are needed for the system startup, then you're obviously right. This is not what I'm referring to though. The containers I'm thinking about are data containers. They can be mounted at will, like the containers that are currently supported by the plugin.
For those, I propose a new feature: configure the name (full path) of a key file, with which the container is encrypted. So long as they key file is absent, the status is as it is now (no unlocking). Once the file becomes available (say, a USB drive has been mounted), the container is auto-unlocked.