Psychotherapy is a highly collaborative and individualized mental health practice developed in (post-) modern societies. The mental health outcomes of psychotherapy cover a broad range of psychological factors including the reduction of suffering/symptoms as well as the promotion of well-being, personal values, and personal strengths. There is extensive meta-analytic evidence that legitimate psychotherapy works remarkably well and robustly for most common mental disorders. In addition, there is a large body of meta-analytic evidence supporting the potential relevance of transdiagnostic relationship principles and transtheoretical psychotherapy factors. Based on this ongoing empirical evidence, we propose four relevant implications for future training and practice in transdiagnostic psychotherapy: 1) the development of a transtheoretical legal framework for psychotherapeutic treatments, 2) the formulation of evidence-based transtheoretical interpersonal skills, 3) an orientation toward transtheoretical therapeutic factors, and 4) the exploration of comprehensive psychotherapy outcomes. We conclude with some more general guidance for future directions.