Harvester ants don't give new play style or special caste that can be implemented for ereptor colony, they are way too similar to leafcutter so I don't think they'll do it. But I have this idea. In Mexico, Pogonomyrmex occidentalis (western harvester ant) harvest seed from yucca tree, while Myrmecocystus mexicanus (a species of honeypot ant) harvest nectar from yucca sap and yucca aphid, sometimes feed on dead insect. Their nests are usually close to each other. During dry season, these honeypot ants may take dead or injured harvester ants as food (more nutrition than nectar). A level may involve protecting your nest from the fully stored but greedy honeypot ant colony nearby, while having to out-harvest a Dorymyrmex colony under a yucca tree (this species is known for blocking oposing colony with rubble). The burning mid-day heat and lots of night-time creatures will be your challenge. (desert-dwelling harvester ants usually harvest at dusk to prevent this)