We think it might be worth adding a new <notes type=""> element where absolutely any free text about an activity could be added. Also the @type attribute would be free string to allow people to make up what ever attributes they want.
There are already some places where free text is allowed, e.g. descriptions, legacy-data, titles, etc, but there is some ambition from people to included those extra bits of data that help make more sense about an activity that may be don't quite fit the standard as such.
This could result in lots of unstructured data that is hard to use, but might also be useful.
Maybe the attribute becomes too unwieldy over time? Maybe this could be dealt with using a namespace?
What do people think?