Everyone know's the pain of micromanaging six wheelbarrows into a cart to transfer large amounts of resources. This is also a Terra-former's nightmare.
Should be able to use a wheelbarrow to directly place stockpiles into a cart. Once a cart has a certain type of resource, it can then only accept that resource.
The amount of resource it holds would be equivalent to whatever six stockpiles of that item is (aka six wheelbarrows).
Instead of the bags carts normally have, replace it with stockpiles of whatever the resource is, filling up just as stockpiles currently do as far as looks go.
Could maybe click on the cart to get an exact number of how full it is. As well as add/remove individual pieces of the resource.
This is a quality of life suggestion, and would make many tasks much simpler.
I understand there may be concern with people just using these carts as stockpiles from here on out - it's possible.. could add a whole new much more expensive cart specifically designed for this as an incentive to use traditional stockpiling.