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thank you for your interesting question. Yes, as you said, the end users enabled to add vector layers (csv/xls/geojson/kml) can create heat maps and clusters using the User Interface of Spectrum Spatial Analyst application.
Right now, for adding the functionality to create heat maps and clusters to Spectrum Spatial vector layers already in the map - we need to extend the solution. Thanks to the “Integrate” part of Spectrum Spatial we can run a component, the user will be able to select the type of thematic layer to create heat map or cluster, set the opacity and chose the column for using it. Please, see the screenshot below showing one of our extensions of Spectrum Spatial Analyst. The extension is needed because we need to load the data as vector layer from the feature service.
Another possibility is to develop a component adding a button to the UI, when the end user click on the button the component add a vector layer in the map. At this point the user can use the heat map and cluster features of the UI.
We might show a demo of both the above extensions. We can share the codes samples to extend your solution.
Then, we will add this feature in a future release. Andy may add more information about the roadmap.