What I am planning on the rewrite of the Cronjob Package is to allow multiple instances. All crons will have the context_id set to the instance_id of the package instance that created the cron. All privileges are assigned to the instance_id. Essentially the instance_id is the 'group_id'. However, I'm trying to get two modes: user and group. In group mode the package_id would be used to select which crons you can read/write. In user mode, the user_id of the cron creator would be used. You can switch back and forth between modes, per instance.
I don't know how good an idea this is, but I have never found a need for any additional privileges. It seems like the group idea of Jon's would allow any additional specific privs to be created in a very understandable way.
