ArcGIS Portal really needs the capability to create and manage relationship classes in the web interface of ArcGIS Portal. One option is to have a way to create or manage related features or tables in the Settings tab on a feature, or map, service Item Details page. Another option that comes to mind is a new Relationship Class item type that allows you to define the relationship settings in the Settings tab on the Item Details page.
I think the solution should work for both hosted and traditional services. We currently use Relationship Classes in an Enterprise Geodatabase. However, those do not seem to be recognized when using bulk publishing, or if related features are published as two different services and added to a web map or ArcGIS Pro.