Herman - ?
Lily: Joan Bennett
Grandpa: Thayer David
Marilyn: Nancy Barrett
Eddie: David Henesy
I tried this with Star Trek:
The Skipper-William Shatner
The Professor-Leonard Nimoy
Mr. Howell-DeForest Kelly (which I love for some reason)
Mrs. Howell-Majel Barrett
MaryAnn-Yvonne Craig (hey, she guest-starred)
Of course, that leaves James Doohan and George Takei out but it's not like no one else ever visited the island!
Capt. Kirk- Russell Johnson (again I love this idea!)
Mr. Spock-Bob Denver (I think that he would love a dramatic role)
Dr. McCoy-Jim Bacchus
Scotty-Alan Hale, Jr.
Yeoman Rand-Dawn Wells
Nurse Chapel-Natalie Schafer
Sulu-George Takei (can't think of a good replacement)
Chekov-Kurt Russell (did think of a good replacement)
For the Munsters/Dark Shadows, I agree with both Bob and Darrin's choices. (Robert Rodan played Adam, the Frankenstein's monster pastiche, on Dark Shadows.)
I did Star Trek/Gilligan's Island and Gilligan's Island/Star Trek yesterday to post as a follow-up today, but came up with slightly different results.
On the Gilligan's Island side, I cast Grace Lee Whitney as Mary Ann.
On the Star Trek side, I cast Alan Hale as Captain Kirk, Russell Johnson as Dr. McCoy and Jim Backus as Sulu. (No Scotty on my list. Although Alan Hale would make a good one, I prefer him as "Skipper" of the Enterprise.)
For The Munsters/Gilligan's Island, I’d got with…
Herman – Alan Hale
Lily – Tina Louise
Grandpa – Jim Backus
Marilyn – Dawn Wells
Eddie – Bob Denver
Alternately, for The Munsters/Lost in Space…
Herman – Dawson Palmer (LIS stuntman who played most of the monsters)
Lily – June Lockhart
Grandpa – Jonathan Harris
Marilyn – Marta Kristin
Eddie – Billy Mumy
Some sad news about Dawn Wells. From The Huffington Post: "Dawn Wells of ‘Gilligan’s Island’ Says She’s Broke and Asks For Help"