1. Support multiple payment systems without having to switch back and forth
2. Support for Coupon Codes
The call center module requires a lot of changes and needs to be completely rewritten . This is a great piece of work and will be implemented in one of the future versions but not 4.0.
1. Ability to edit an existing order, add products, coupons, etc..
There is already an ability to edit current products in the order along with their price and other settings. But yes ability to add new products to existing order is yet to be implemented.