Knee osteoarthritis (KOA), a common form of arthritis that causes physical disability, has become increasingly prevalent in society, especially among the elders. Employing computer-aided techniques to automatically assess the severity and progression of KOA can be greatly beneficial for KOA treatment and disease management. Particularly, the advancement of X-ray technology and its application in KOA demonstrate its potential for this purpose. Yet, existing X-ray prognosis research generally yields a singular progression severity grade, overlooking the potential visual changes for understanding and explaining the progression outcome. Therefore, in this study, a novel deep generative model is proposed, namely Identity-Consistent Radiographic Diffusion Network (IC-RDN), for multifaceted KOA prognosis encompassing a predicted future knee X-ray scan conditioned on the baseline scan and a future KOA severity grade. Specifically, an identity prior module for the diffusion and a down-stream generative-guided progression prediction module are introduced. Compared to a conventional image-to-image generative model, identity priors regularize and guide the diffusion to focus more on the clinical nuances related to the prognosis, based on a contrastive learning strategy. The progression prediction module utilizes both forecasted and baseline knee scans, and a more comprehensive formulation of KOA severity progression grading is expected. Extensive experiments on a widely used public dataset, OAI, demonstrate the effectiveness of the proposed method.