If the Gilligan's Island series had continued for another 10-15 years, would we have seen the next generation appear? Would Mary Ann or Ginger have a child?

Yes, both fathered by the Skipper’s “little buddy”.

The professor is the only one attractive. He was clueless though.
I doubt either woman still in child bearing age would want to have a baby where virtually no medical care existed. The prospect of an infant surviving would be low. What kind of life would it be? Even if both women had children, none of them could have a mate. It would create a circle of offspring who could only expect inbreds for a next generation.
It just wouldn’t be feasible.
Now, messing with Gilligan just for the fun of seeing him all worked up might be good for entertainment.

As described here the characters on Gilligan’s Island represent the Seven Deadly Sins.
The Professor = PRIDE
Mary Ann = ENVY
Ginger = LUST
Mr. Howell = GREED
Mrs. Howell = SLOTH
Skipper = GLUTTONY and ANGER
Gilligan represents Satan.

So it’s very plausible that both Ginger and Mary Ann get knocked up. Excluding Gilligan, since that presents a Rosemary’s baby situation, I would guess the Professor would be the father of both.
The Skipper and Mr. Howell are probably impotent, and I doubt the Skipper’s sexual preference would interest him in the farm girl or the bathing beauty.

Or maybe Ginger and Mary Ann discover each other. Nobody ever sees any proof, but from time to time the cries can be heard from somewhere in the depths of the jungle growth. Or are there kookaburras on that island?

I always thought that the Professor & Mary Ann would get married & have several little geniuses!!!