Glad to see this feature request already submitted, disappointed that it doesn't have more upvotes... We want to include custom fields that pull values from the user table and teams table so reports and filters can then be designed around the custom field using wildcard filters.
Use case #1: - Create a RACI matrix for tasks that can be reported against. We can rename the 'Assigned To:' column to 'Responsible' and that column is easily reportable, but there's no way to report "Accountable" "Consulted" or "Informed" equals $$USER.ID. Additionally, these fields combined with an OR operator could be used to generate a Project Stakeholder list.
Use case #2: - Select a user or team from a drop-down field (or text field) in a custom form. E.g. "Preferred Resource" that allows a user to request (but not assign) a specific resource in their submission. A report can then be designed for users/teams where "Preferred Resource" equals $$USER.ID OR $$USER.homeGroupID
I predict there could be value in expanding custom form validation against other tables, like project or program, but users and teams would be a great start.