HDMI connectivity would require the DSP section out of a surround processor to be useful. It would also tend to require other inputs to allow the unit to work with non-HDMI devices, at which point you've built in a surround processor. Once you build in the amps for the sound bars speakers, you've actually built a complete surround receiver into it. That will get kind of bulky and complicated.

What about a sound bar with just center and the surround deflection speakers?