Currently, when adding an activity into a "slot" of a given owner activity, the designer always creates a new Flowchart activity and opens it in the designer. Although this allows for deep-nesting components, it will also become harder to retain a proper overview of the process as a whole.

What's missing is a sequential activity designer where a given host activity displays its children inline.
For example, the If activity has two slot: Then and Else. Currently, the contents of these slots are hidden away.
A sequential designer should render host activities and their children as nested components, and not hidden as is the case with the flowchart designer.

This would work similar to the Windows Workflow Foundation set of designers for Flowchart and Sequence.

Example of a WF flowchart designer:

Example of a WF sequence designer: