Bipolar disorder (BD) is a serious mental illness that requires life-long management. Manic and depressive mood episodes in BD are characterized by idiosyncratic behavioral changes. Identifying these early-warning signs is critical for effective illness management. However, there are unique design constraints for technologies focusing on preemptive assessment and intervention in BD given the need for data-intensive monitoring and balancing user agency. In this paper, we aim to establish acceptance, needs, and concerns regarding a preemptive assessment and intervention system to support longitudinal BD management. We interviewed 10 individuals living with BD. To ground the findings in lived experiences, we used a hypothetical assessment and intervention system focusing on online behaviors. Based on the data, we have identified requirements for effective behavioral monitoring across illness episodes. We have also established design recommendations to support dynamic, longitudinal interventions that can address the evolving user needs for life-long BD management.