Well, you thought that harvesters have special/defensive powers. I thought of making each commander's harvesters unique... as in Seth has a harvester that's different from Oxanna and Kane has one that's different compared to Solomon.
Strong-arm has a harvester with a machine gun.
Dr Liang's harvester has a built in healing station that heals slower compared to his drone.
Solomon's harvester has a Ion cannon blaster.
Jackson has a harvester with 15% speed and 10% harvest rate.
Seth has a burrowing harvester (When spawned, it instantly burrows to the closest tiberium field, appearing on the field).
Kane has a harvester with an obelisk on top.
Jade has an invisible harvester powered by tiberium. It only turns invisible if its on a field and a catalyst missile destroys the harvester instantly, leaving a 19 tile explosion with 2x more damage compared to a regular tiberium explosion.
Oxanna has a harvester similar to jackson's, but it has 20% speed, 15% harvest rate and 10% vulnerability to damage.
All harvesters that can defend themselves will only attack their enemy if the enemy attacks them. Solomon's harvester attacks like a titan (but with half the DPS), while Strongarm's and Kane's harvesters work like their defensive towers, respectively(also half the DPS).