An annotation is a note that is made while reading any form of text. This may be as simple as underlining or highlighting passages. There are no standards in this field, but there have been some developments already (Open Annotation Data Model). WS could implement a solution like hypothes.is, Textus, or develop its own.
If successful, it could be used as a basis for the Wikisource Annotation system. Afterwards, annotations in Wikisource could be linked/referenced/transcluded in Wikipedia.