Hi Karol,
Currently there are no plans in WF 4 to make these types inheritable (actually, all activities in the framework are sealed). The approach that we recommend is using composition (activities can be freely composed) and when that is not enough write your own custom activities. The general model that WF has taken is one that favors composition over inheritance.
That said, the story for writing custom activities have been dramatically simplified in .NET 4. There are no special steps involved in writing a custom composite activity (in WF 4 is just an activity like any other else). We are providing the activity authors the same tools that we use for writing the out of the box activities (we are not using any special or internal feature in Sequence or Flowchart).
Thanks,
Leon Welicki,
Program Manager, Connected Framework Team
