I would like to see the possibility to filter by some user properties in constraints. This could be used in comparisons of the current user and entries in specific user fields, or to check if a user belongs to a specific group/dynamic group.
Example 1: a constraint should prevent a status change if the current user is the same as the user which creates this item.
Example 2: the same constraint as in example 1, except the current user is member of the admin group.
At the moment we have to use triggers to fulfill similar requirements.