Extend the history data so that it can be used for audit trails. Essentially, the engine needs to be able to show who submitted what data in certain task forms. More generically, this means that people should be able to inspect who has fed which data into the process. This applies mostly to users that enter data in a user task, but also for external systems that are invoked or signal process execution.
The first aspect is that the Activiti Engine can manage this information. Secondly, Activiti Explorer should be able to present this information.