Therapeutic games can be considered as a promising rehabilitation tool since they provide personalized rehabilitation sessions in which training intensity and challenges can be adapted to the patient's ability, motivation and performance. In this paper, we discuss post-stroke therapeutic game design challenges and requirements that open-up a variety of adaptation issues. Our objective is to present a generic difficulty adaptation technique for a family of post-stroke upper-limb rehabilitation games. The proposed technique aims to increase rehabilitation volume by maintaining patients' motivation and engagement.