I think it would be nice to see a virtualized Voodoo5 as well as a virtualized and modified Geforce FX with full speed FP32 shader precision and RGSS. The Geforce FX only had ordered grid and half speed FP32 pixel shader speed; the former especially sucked to me, and they had to use half precision fragment shaders so that people wouldn't bitch about low framerates.
I always wondered why games didn't default to FP16. It was years before games came out who's shaders really needed the higher precision, and the FPS difference was substantial. Most could be forced to 16, and looked just fine.
Not sure why you'd want a GF FX virtualized, though. I'd rather just see old D3D features better supported, with compatibility code that was separated from modern performance code (IE, DX7/8/9->DX11 wrapper, adding compatibility-optimized code for features no longer existing in HW). High quality texture filtering and SSAA (I like 4xSGSSAA, when it works) or mixed MSAA+SSAA look great for old games, that still work, and unlike modern games, performs well enough. If you want better SSAA, and functionality not there anymore, just get an old Radeon, and the legacy Catalysts.
Now, a virtualized Voodoo 5, for a retro gaming rig, would be pretty damn neat.