[Tom Bunch]

We can get you pretty close.

Under the View menu we have a "Focus" option. This will not technically changing the root of the tree, because it will, to use your example, keep the ancestors of the task you're focusing on in the tree for context:

* Task B
** Subtask 2
*** Sub_SubTask a)
**** Activity 1
**** Activity 2
**** Activity 3

Many people use a hierarchical naming scheme where tasks might recur in multiple phases of the project and the names don't mean much without the context of the parent tasks. So until we figure out some way that you can have that context present in all configurations of all views, we opted to keep the ancestors of the focused task in the tree, too.

-Tom