P.S. I really enjoyed Jumanji as well. The studio bailed on the sequel so the creators had to rewrite what was supposed to be the sequel....and that became Zathura.
It was argued about during filming whether it was a true sequel or not - it is based on another work by the same author.
Both Jumanji and Zathura are based on novels by Chris Van Allsburg; in the book version of Zathura, the Zathura board game is found inside Jumanji, but an astronaut living inside Zathura claims to have created Jumanji.
Many have settled on calling it a spiritual sequel: a successor to a work of fiction which does not directly build upon the storyline established by a previous work as do most traditional prequels or sequels, but nevertheless features many of the same elements, themes, and styles as its source material.