I dunno. Part of what made Chef "Chef" was that we the audience knew it was Issac Hayes doing the voice. That was the "joke".
Replacing Sheila Broflovsky's voice is one thing- no one knew who the actress was. Replacing Chef's voice with someone doing an Issac Hayes impersonation will just sound like someone doing an Issac Hayes impersonation.
I wouldn't be surprised if Chef has a deep-voiced brother/son/cousin, etc who suddenly shows up in South Park to dispense wisdom to "the children".
I could be wrong. But, if I'm right? You heard it here first, baby!