The problems:
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The optimal strategy is to park units outside of your opponents base, and harass there workers. There is no drawback to this, and it leads to games where the game is decided by the first one to build a tower near the enemies gold expansion, and stock it with spearmen.
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Units can park for long durations and be a persistent threat for no cost deployed far into enemy territory. This can decide a match easily, and prevents the defender from having a chance to get back on their feet. This results in matches that result in an outright steamroll instead of a push/pull scenario.
The solution:
The resupply mechanic. All units now have a metric called supply, similar to health. When supply is maxed out, the unit begins to passively heal over time (very slowly, similiar to scout healing) However, when units are not near a structure or unit that can restock their supply, the supply begins to dwindle over time. When supply hits zero, the unit suffers. Their attack speed and damage is decreased, they begin to lose health, and they are unable to use ranged attacks.
So, what can refil supply? Well, mills, town centers, and mongol gathering structures can refill supply of all units. Supply trains can refill supply of all units minus supply trains, and light supply trains can refill infantry units, but not cavalry or siege units.
All supply trains can be trained at the town center, and light supply trains start in the dark age, while normal supply trains start in feudal.
Certain units burn through their supply more slowly. All supply trains burn through supply 10x more slowly than normal units, and the scout does not burn through supply at all. Certain techs can decrease the rate certain units burn through supply.
So what does this do? Well, if you can destroy an attackers supply train, he may be forced to call off the attack in 1 minute, rather than 10 minutes later. It also gives a reason to build forward mills/farms/outposts in order to extend the reach of your empire. It add strategic depth to the game, and it gives the defenders a chance to push back, which means games are less of a steamroll, and more of a push/pull. I hope you will consider this mechanic and support it. 