If you mean the 13 Ghosts of Scooby Doo, you're not the only one who has been wanting that; actually i've had a few good what if's as far as Scooby Doo is concerned, some of which may have been discussed her before but here goes:
What if Fred and Velma were in the cast of the 13 Ghosts of Scooby Doo and there was no Scrappy in that series as well?(I actually did a fan fiction series with that concept)
What if the 13 Ghosts of Scooby Doo series had gotten a longer run and the gang actually caught every ghost from the Demon Chest?
What if in 1979 instead of Scrappy, a character like Madelyn Dinkley(From Scooby Doo Abracadabra Doo) was introduced to the Scooby series, and that character would travel with the gang(basically I think this would have worked because you would have had a female in the 1980-1982 seasons and you would have had someone who was attracted to Shaggy, which means maybe there wouldn't have been a character like Googie introduced later)
What if when Fred Silverman left ABC to take the job as the head of NBC programming, he did the same thing he did when he left CBS to go to ABC, mainly to bring Scooby Doo along with him, in other words what if Scooby Doo aired on NBC's saturday morning schedule?
What if Scooby Doo had aired in the 1986-87 and 1987-88 seasons on ABC and if so what would the show have been like?
What if Scooby Doo Mystery Inc was made a few years ago and aired instead of Shaggy and Scooby Doo Get A Clue?
What if Fred, Daphne and Velma were around for the three syndicated movies in 1987-1988(recluctant werewolf, Ghoul School and Boo Brothers)?
What if the original idea for Scooby Doo(Mysteries Five) after a decade or so was dusted off and started airing on CBS, ABC, or NBC saturday morning schedules?
Finally(at least for now) one that would have been very interesting: What if instead of Scooby Doo on Zombie Island being a movie, it would have been like a mini-series or the springboard for a new Saturday Morning or Cartoon Network series and in the final episode of the series, Fred and Daphne would have a wedding, and maybe Shaggy and Velma would have a wedding as well?
Those are the only ones I have for now, but I have plenty more interesting ones that I might post later