What would be the role of socialit planning in a worker coop based society? What can we learn from the Soviet experience ?
Much in the Soviet planning systems (they changed across the 1917-1989 period of the USSR's existence) and in variations that other countries (both socialist and not) experimented with can and would be brought to bear in a new economic system that comprised a planned distribution of resources and products plus a worker coop organization of enterprises. You can already see some of what this could be in Spain's Mondragon Cooperative Corporation. This is because it is a holding company, a family of over 200 worker coops who undertake all sorts of shared, coordinated activities such as a bank and a university. MCC also has rules governing how unemployment suffered by any one member coop is responded to and shared by other worker coops. The goal here is the strength and solidarity of the totality of MCC upon which each worker coop depends and thus to which each contributes. This would similarly be the goal in a socialistically planned economy of worker coops which would include, of course, a much larger number of worker coops (although there might be interest in and argument for grouping them in industrial or regional sub-groupings. Socialist planning would govern how resources are allocated to each enterprise and how products flow from enterprise to enterprise and from enterprises to the public. Finally, there would be room for use of markets (most socialist planning systems have allowed for markets while subjecting them to all sorts of regulations much as they have been regulated in other ways in capitalist systems.
Perhaps an important basic point: our focus on worker coops was never meant to suggest an alternative definition of or task for socialism: the idea was always to correct and complete the initial experiments with socialist economies (USSR, PRC, etc) of the 20th century by ADDING and INTEGRATING the democratic reorganization of enterprises into worker coops.
You might find worthwhile S. Resnick and R. Wolff, Class Theory and History: Capitalism, Communism and the USSR New York: Routledge, 2002.