Description

Workflowitems shall be used as follows: If user x assigns user y, user y gets permissions automatically and user x' write/admin permissions are revoked. So only the assignee can change the workflowitem. If the assignee changes the assignee's permissions changes too.
The assignment should happen after a short confirmation to prevent unintentionally assigments.

Solutions

Typed Workflowitems

Workflowitems can have specific types when creating them. These types are described in the docs or in the creation dialog. Each type has spefic behavior when e.g. assigning a user or closing the wfitem. For this feature request a type like e.g. "restricted" shall be added which behave like described above.
A user can choose the type of a workflowitem via dropdown at workflowitem creation.
At first these types shall be implemented in frontend only.
For Trubudget 2.0 these types can be specified on api/blockchain level.

Implement as general behavior

Every Workflowitem should behavior like described above. Trubudget should be as general as possible so it's not possible to implement this solution

Other Solutions

...brainstorming needed