One thing I'd want to see is support for standalone polyglot utilities, i.e. that you can write utilities in any language and they compose seamlessly into the system because they are all launched as independent processes. That is one thing I would like to retain from classic unix-style shells.

The problem of designing the shell becomes both much harder and simpler at the same time. Harder because you need to figure a way to really pass objects from one process to another, which implies some sort of serialization protocol. On the other hand the shell becomes simpler again, because the processes will (most likely) do most of the heavy lifting of managing objects etc.

Powershell 6 is open and runs on Unix.