Currenty Legend supports the following fields:

Other information as tags and contacts are somehow parsed from the text entry itself following a specific syntax.

Those fields are internal and only made partially visible in the entry text, if you right click on the entry or in the group by panes.

So architecturally it would be really great to add 2 functionalities to Legend in the longer term:

Support having additional fixed fields as well as the ability to add user customizable fields.
Aggregation logic that would provide automatic calculation on a parent item based on it’s children items.

Examples of fields:

The aggregation of numeric fields would simply summarize the values of the children items.
Examples:

Estimated Duration would be the duration from the earliest child start date/time to the latest child finish date/time.
Cost and Effort would simply be sums of all the children items