Jimmy Fallon does good vocal impersonations, though.
SNL will never be truly great without Bill Murray, Billy Crystal, Dan Akroyd, Victoria Jackson, etc...
The last really good performer from SNL was probably Will Ferrel.
At any rate, Al would be excellent on Mad TV or SNL (if SNL was still doing Celebrity Jeopardy, I'd love to see Al impersonating Eminem

), but I think it would be even cooler if he got his own sketch show...a new season of TWAS, but with a different format...more sketches, no lesson, etc.
But that's not likely to happen anytime soon.