I wonder if there is any way to represent a polygon feature in the legend with a custom symbol. I can think of several ways but the problem is that the legend must be dynamic, only showing objects visible in the map extent, and therefore not converted into graphics. Also, the legend should be included in the layout template for these standardized maps. One additional problem is that the data must be contained in the enterprise database and that database can't be manipulated or complemented with additional data.
The polygon layers represents small features why the default symbol of a polygon (a filled square) constitutes a poor representation of the map features.
My idea is creating a second data frame in the layout template with an extent that follows the primary data frame, only showing the necessary features. The symbology of these features could then be manipulated to give a good representation in the legend which is created from the second data frame.
But yet, the polygon symbols are still hard to manipulate in order to give a good representation in the legend.
One example would be rivers created as polygon features. A blue box in the legend gives a quite poor representation.