The Jimmy Neutron Revival Project is a project led by many fans of the Jimmy Neutron franchise in order to get Nickelodeon to bring it back for new adventures and audiences.
A common drive to achieve a revival is to see the resolution to the many plots, and the cliffhangers the show left viewers on when it was ended right before its final season. These include (but are not limited to) Jimmy and Cindy's relationship, the state of Retroville's security from frequent villains, and developing friendships between characters.
The revival project actually began as far back as 2004 when season three started airing it's episodes out of order in the US and therefore, viewing and ratings started to decline. After all of season three aired, DNA Productions shut down due to the failure of The Ant Bully and cancelling Jimmy was a big loss for the company and they all were out of jobs.
If season four did come to be, Evil Jimmy would've returned and officially defeated, the League of Villains would've made more appearances, the villain humor would be kept to a minimum, Nick would've rejoined the main cast, there'd be an overarching story about the gang starting middle school, Cindy and Betty would've become friends, new villains would be introduced and Jimmy and Cindy would've finally stopped pretending to hate each other.
In 2008, a spin off series staring Sheen was greenlit for 26 episodes and it premiered two years later, but it ended in one year due to low ratings and negative reception.
Keith Alcorn (one of the creators) has also spent about two years trying to bring Jimmy back as well and he and several other former staff members (along with others: Ryan Fortier and Edward Pace, who proposed many ideas for season four) have made four scripts for new episodes, but have kept them in secret until further notice.
With Nick's new decision to revive most of their old materials (which right now are; Invader Zim, Hey Arnold, Ren and Stimpy and Rocko's Modern Life), the same could still happen for Jimmy Neutron.