Piwik already tracks everything needed to give a good overview over evolution of performance of a store.
What is missing is, doing some simple math with the values and show them beside the already available ones in ecommerce evolution over the period graph.
These values often are used to get a quick overview and are actually (2.13b1) missing in the graph:
and in addition not available within piwik today, but very very important for this overview:
Thanks @hpvd for those suggestion of new Ecommerce metrics...
maybe we could start with those for which the data is already archived and we can process those:
purchased products/Order (average number of purchased products per order)
revenue per visitor
ratio abandoned carts/ordered Carts