Built-in simple or advance ecommerce system management
Built-in ecommerce system like shopping, payment. So that joomla will also be called as CMS as well as ecommerce management system.
Thus the user will use the inbuilt core functionality, with using third party components.
Joomla should at least have basic integration to paypal or google checkout out of the box...
Morgan Skilling commented
As an ecommerce developer I can tell you that's a slippery slope to start going down. You think it too Joomla a long time an a gazillion work hours to get where it is today - that's about what it would take to do ecommerce as part of the core and do it right.
There are some nice ecomm components already out there and more no doubt will come along.
Use VirtueMart. I think it is enough. Don´t overload the core.