I think adding in some sort of hook part into the game for attaching cargo to a craft. This part would be roughly the same size as the fuel adapters but function differently. I have noticed that the astronauts have a tether which would make developing this easy. Instead of a tether, the astronaut can walk up to 2 crafts that both have hooks/latches and connect them to the main craft.
If you have a cargo plane and you want to transport something, connecting it through a docking port or an interstage is annoying. When the two crafts are connected the cargo would become fully connected to the craft and would not even move. The cargo would become completely stationary.