Since August 31 it is possible to compose string variables, which is great. However, I miss more options:
1) Compose other variable types, mainly
integer
float
boolean
arrays
2) manipulate them with WDL (https://msdn.microsoft.com/en-us/library/azure/mt643789.aspx), as it is currently supported when editing Conditions in advanced mode
This way lots of use cases that require counters, string manipulation etc could be implemented directly with Microsoft Flow
You can use any of the 70+ expressions from the Workflow Definition Language in Flow: https://flow.microsoft.com/blog/use-expressions-in-actions/