In Desktop software, we have the Make Query Layer tool that allows us to perform all sorts dynamic joining and filtering of Geodatabase data to visualize data from multiple feature classes or tables without having to duplicate the data storage. I would find it incredibly handy to have the same capability within web maps using Arcade FeatureSets. If we can define a FeatureSet in popups and label expressions and use them to dynamically access data from multiple layers, could we take it one more step and add functionality to give a map creator the option to say "Add Arcade Layer"? I'm thinking there would need to be requirements that only one spatial field be included in the output and that all the geometries in the final spatial field have consistent coordinate systems etc. But I can see great benefit in being able to add a layer to a map that (for example) combines features from multiple point feature service layers that all have a common status that needs attention so that they could be shown as a single layer in a web map. I can think of many other scenarios that would be useful here too and I would say that there's justification for it because of all the same reasons the "Make Query Layer" tool exists.
