Alter a defined featureset

I would like to be able to alter a defined featureset in a workflow. Similar to how you can set a collection to "union, replace, subtract or intersect" I'd like to be able to do the same with a featureset.

The current methods of modifying an existing selection in html5 are, to be blunt, utterly woeful. As a result we have to write workflows to do this basic GIS function. even this has proved horribly clunky if you want to then use the selection in another workflow.