Gregg,
That's a brilliant idea and I am completely dumbstruck as to a suggestion. OK, well, let me throw out a few probably obvious choices, with apologies if I've forgotten multiple female roles.
Duality
Why? Simon Radford. Be interested to see someone else's take on that very memorable character.
When Darkness Falls
Why? A show focused more on the secondary characters. Be interested to hear someone else interpreting Harry Kelly.
Secret Origins
Why? Geez, you gotta ask?

The World Next Door
Why? To see who else might be up to the challenge of Baboon McSmoothie!
No Justice
Why? Trixie and Sabien? What a great combo.
Mixed Blessings
Just an all-around good episode.
Sabien's Law
Lots of Sabien.
Hm, I guess maybe not so dumbstruck.
The current casting seems so perfect, it'll be interesting to hear these fantastic roles voiced by someone else.
Bill