I wonder how many of these actually came close to happening because I can see just about every one of them in those prospective roles.
Jerry Van Dyke was the first choice to play Gilligan. He was sent a script by Sherwood Schwartz, but dismissed it as "the worst script he ever read". Van Dyke did MY MOTHER THE CAR instead, which lasted a single season, and is routinely cited as one of the worst sitcoms ever. In Van Dyke's words, "I've had longer showers" than the time MY MOTHER THE CAR was on the air.
Carroll O'Connor, Raquel Welch, and Dabney Coleman were apparently tested during the open auditions. O'Connor and Coleman were apparently strong contenders, albeit briefly, while Welch was dismissed as "too sexy" for Mary Ann. Dawn Wells once said, "she would've made a better Ginger" rather than being a good Mary Ann.
I'm divided on how seriously Mansfield was considered for Ginger. I've heard it repeated often (and I've said it myself on numerous occasions), but I do doubt the legitimacy of the claims. Mansfield was still a big enough star in 1963 (when casting would've been done) that taking a supporting role in what is basically an ensemble sitcom would've been viewed as a "step down" for her that wouldn't've seemed logical for her to do during the era. Television was still considered a "lesser medium" in comparison to film. However, I've seen a few biographies mention she was offered the role (such as the A&E "Loves & Kisses" TV bio), and Bob Denver even mentions it in the 2001 made-for-TV docudrama SURVIVING GILLIGAN'S ISLAND.
So, who knows? She may have been considered, but either way, she didn't end up doing it. The most common perception is that Mansfield declined the role because she was a movie star that didn't want to do a regular TV series. Which, I guess is logical, but goodness knows she did a lot of TV, especially after her movie career folded beneath her.