Directed evolution of cells is generally done by setting up a screening process. An example is a binding assay for evolving cell surface receptors.
It is very difficult to build screening procedures that select for functions. However, it might be possible to engineer "killer" cells that attack cells with particular types of behaviors. Thus the killer cells provide the screening process. And why limit to a single type of killer cell.. of course, the killer cells should not evolve (might be an issue)
Imagine this scenario: a population of cells evolving in an environment with populations of 3 or 4 different types of killer cells. Each killer cell targets a particular type of behavior. Further, another population of "helper" cells excrete specific nutrients in response to particular behaviors. The target population of cells should evolve to avoid specific functions that are targeted by the killer cells and acquire specific functions targeted by the helper cells. Due to the existence of multiple criteria, the evolution might be more gradual as well.