Tracking difference in resources lost between you and the opponent
At the moment, none of the metrics really measure army control, tactics, army composition, scouting, and map awareness. However, if there was a metric that was tracking the difference of resources lost between you and the opponent, this could be used as a proxy for these, as good control/composition/scouting/tactics/etc means you take better fights and lose less resrources than your opponent. Tracking this between games and matchups would be nice to motivate players to take better battles. Similar to this, workers lost would be a nice metric as it can be used to explain map awareness and scouting cause you're less likely to lose workers if you spot harass in time
Robert Howard commented
There is an option on the graph for resources lost that I use for this and workers killed is on the front page too. It has yielded some nice insights. In most games I can see loss momentum gaining or losing ground especially in mirror matchups Are you talking about tracking these stats long term instead of just during a specific game?
A simple metric would be average of opponent's resources lost divided by the average of my resources lost. 0.5 means I lost twice as much as my opponent. 2 would mean my opponent lost twice at much as me. And the user wants to maximise this number which should be done by taking better engagements, having better army control, scouting better, etc.