Co-op missions with bridges that deploy/form from the campaigns.
This changes the terrain a bit, allowing more ways to get around the map.
Do you miss those missions with that? Seeing those bridges form was like a little eye candy :).
Of course, it would be needed to trigger those bridges… the bonus objective might me a good choice (or make the bonus harder to kill), or destroy certain targets, or be in a certain location (that happens in Lock and Load, Miner Evacuation, Cradle of Death)… or simply find the button to activate the bridge, like in Warcraft 3.
What do you think?