Can additional capabilities be added to the base ArcGIS Enterprise deployment?

Yes. Sidwell offers additional configuration options for ArcGIS Enterprise including: ArcGIS Image Server, ArcGIS GeoAnalytics Server, ArcGIS Field Apps, ArcGIS Hub, ArcGIS Monitor, ArcGIS Insights, ArcGIS Dashboards, and Application  development (stand up apps, scripting, etc.).