Subgrids are great but the limitation of needing a direct relationship between the sub entity and the main entity makes it very restrictive. There are many different use cases, if instead of using a pre-configured view that only allows linking to the active Record via a predefined relationship, we could instead bind a custom FetchXML do a subgrid and this custom FetchXML would allow for one token called #RecordID# to reference the currently open record on the form by returning the GUID.
Use Cases of Subgrids that could be placed on an account if we could filter based on the active record:
fully support subgrids that include records down the hierarchy
e.g. show opportunities of this account and all accounts lower in the hierarchy (Opportunities WHERE AccountID = #RecordID# OR AccountID.AccountID UNDER #RecordID#)
fully support subgrids showing data that is connected via multiple relationships
e.g. show all orders that have the AccountID of the active record in Ship To Account OR Bill To Account (Orders WHERE BillToAccount = #RecordID# OR ShipToAccount = #RecordID#
fully support subgrids going down 2 or 3 or n levels instead of just the one direct level possible out of the box
e.g. show all event registrations off Contacts at this Account (Event Registrations WHERE Contact.ParentAccount = #RecordID#)
Please either make this available as
Solution Option A: via a direct customization option in PowerApps (instead of selecting a view for the subgrid, allow the user to copy/paste a fetchXML query into a text field that can contain a #RecordID# token to link back to the GUID of the record of the currently open form)
In a perfect world you support both Option A + B
There are hundreds of forum entries around the web using unsupported hacks to achieve the same results.
In Classic UI people used the subgrid.control.SetParameter("fetchxml") function but this is now broken in Unified Interface. In Unified Interface it appears a Retrieve Multiple Plugin is the only way to achieve this which is extremely complicated for something as simple as binding a query to a subgrid.