I thought Ghoul Patrol was the sequel. The universe is shattered!
There is also that unlicensed Chinese Genesis game called Ling Huan Daoshi (or Elf Wor in the header).
The game is strikingly similar to and uses the Zombies engine, though has a more eastern setting:
The Zombies team later went on to create Herc's Adventures (Saturn is the native and best port of that):
It's considered to be the in-official third game in the series, because of the very similar game-play and humor.
Edit: And great hack, Zombies The Sequel, good work! You can never have enough zombies.