IMHO having the capability to group Activities in “Processors” can help reduce the visual impact of complex microflows and improve reusability (similarly to the role of Building Blocks).
This kind of grouping (differently from sub-microflows) will allow having different input and output connections entering and exiting them.
For example, Apache NiFi allows something similar through the definition of Input and Output ports. Ports are typed and can accept or return different parameters.