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We design an advanced reflective-diffractive optical element (DOE) for high-resolution, parallelized soft x-ray spectroscopy of, e.g., transition metals, here optimized for the Fe L3-edge around 706.8 eV, and demonstrate its functionality in simulations. Based on a "hybrid" reflection zone plate (RZP) on a spherical substrate, which maintains aberration-corrected focusing under a wide angular acceptance of 8.5 msr, slightly slanted grating grooves transpose the wavelength dispersion in the (-1)st diffraction order from the vertical (z) direction partially into the horizontal (y) direction, and the signal can be read out at MHz rates from a pixelated (50 μm) line detector behind an exit slit. As an advantage over conventional variable line-space gratings of equal aperture, constant resolving power is achieved across the full range of 695-715 eV. The transmission efficiency Pt and the energy resolution ΔE scale in proportion to the adjustable slit width Δh. For a source of (1.0 × 0.3) μm2 in size and Δh = 5 μm, we expect from simulations Pt ≈ 0.3% and ΔE ≈ 0.26 eV, with great potential for their ratio Pt/ΔE to be tuned. The substrate curvature of 4.5 m and the line densities ≲7.19 × 102 mm-1 are compatible with the fabrication of the Ni-coated RZP by laser lithography. In-vacuum and tabletop operation are supported by an optical path length of 490 mm. We derive the vector field of the DOE, simulate its optical function, and characterize the performance for a variable source size. Conceptual extensions for laboratories and large-scale facilities are discussed.