The present study investigates the application of a thin downstream fixed bluff body (TDB) as a passive flow-control strategy to enhance the performance and self-excitation capability of a bladeless wind turbine operating under flow-induced vibration (FIV). Unlike conventional tandem arrangements with a symmetric TDB, where the oscillating mast requires an initial cross-flow disturbance to trigger vibrations, the proposed asymmetric configuration inherently promotes self-excited oscillations through wake reorganization. This mechanism not only enhances the oscillation amplitude and energy harvesting performance but also enables reliable self-starting and sustained operation under unsteady flow conditions. A series of comprehensive experiments was conducted on fifteen TDB geometries grouped according to cross-sectional shape, in addition to five aspect ratios (AR) and spacing ratios. The effects of these parameters were evaluated through vibration response, power generation, flow visualization, particle image velocimetry (PIV), proper orthogonal decomposition (POD), pressure recovery, Reynolds shear stress, velocity deficit, turbulent kinetic energy, and enstrophy analyses. The results demonstrate that the introduction of a properly designed TDB fundamentally reorganizes the wake dynamics. An optimum spacing range of 0.3 ≤ Z/D ≤ 0.4 and an aspect ratio of AR = 0.17 were identified, resulting in voltage and power enhancements of approximately 56% and 81%, respectively, compared with the baseline configuration without a TDB. Among all investigated geometries, the asymmetric TDB exhibited the best overall performance, producing approximately 12% higher voltage and 15.5% higher power than the primary rectangular TDB while significantly improving the self-excitation characteristics of the system. Flow-field analyses revealed enhanced wake confinement, increased circulation and vorticity, stronger coherent vortex interaction, delayed pressure recovery, and greater concentration of energy within low-order POD modes. Enstrophy analysis further showed reduced mean enstrophy and transport accompanied by increased fluctuating enstrophy, indicating a transition toward a more coherent and energetically efficient wake. These findings demonstrate that asymmetric downstream bluff-body design provides an effective approach for enhancing fluid–structure energy transfer and improving the viability of bladeless wind energy harvesting technologies.