As of now, the fields in Salesforce are being copied and rendered into the drop down list making customers wondering if they could display other information from the object as well.
Quoting an example,
In Task, you have task ID and other related task fields but no Account or Case fields in the Task. The drop down list should display the list of fields which are only in Task instead of showing Account ID, Case ID and other related fields which are not in Task object.
If there's custom fields from salesforce then, the fields should be shown else it should just merely based what the layout has.