Have a Power Query action that is able to use the outputs of other actions in the current Flow. This would be the ability to leverage Power Query to aid in the ETL process of Flows including leveraging Gateways to pull on prem data, joining outputs of other Actions in the current Flow, and the ability to generate data directly from the Power Query Action (date tables, random numbers, hardcoded tables, etc.).
This would be a large improvement on things as simple as imitating a SQL left outer join in a Flow or handling "merges" between data.
To aid in the UI not being able to see data the step could use data from a previous run, user entered "example" data, or just allow for M-Code to be written/pasted into the Power Query Advanced Editor.