When you are building a new project, or building a Template, it would be handy to Assign Tasks to a placeholder... e.g.: This task is to be performed by a TAX person... we do not yet know 'who', but the TAX placeholder reminds us the kind of individual to assign...
This would be especially handy for Templates that are needed but not used all the time... right in the straight definition of the Task, we are reminded what kind of individual we need of each Task.
When creating the project, you review the assignment needs, having the placeholder reminders for the work. After you know 'who', the placeholder is replaced by the actual assignee, who will be responsible / accountable for that Task.
This list of 'acceptable' placeholders could be maintained in a user configurable list... allowing for updating / maintaining...and supporting drop-down control for selection... The Assignment lookup would allow selection of either a real user for actual assignment or selection of a placeholder for later actual assignment.
Please let me know if you have any questions