Multiple users on one case

I think this will require schema change, for example adding an extra table to map nid and uid's.

It sounds you describe actually a project. treating it as such means that every 'case' (i.e. mini-project) can have several maintainers.

Changing the CT design so that you could assign multiple users to 1 case would demand a huge design change.

This will not be done in the near future.

Case Tracker is from the start designed as an Case (Issue) Tracker and not as an project management tool.
You could create some basic project management by creating new node types with the CCK module and define them as case node types. But you still won't be able to assign multiple users to 1 case.

We have gotten around this by creating some fake users that represent emails that forward to the appropriate people in a team. For example we can sending editing tickets to the editing team and then they move the assignment to the person that takes the ticket when they update the case. Hope this makes sense.