Though it's been a year since the show wasn't renewed (aka worse than cancelation), the loss of the rebooted Thundercats (2011-2012) series still strikes a nerve with fans. MTV Geek covered a Thundercats panel at at a con headed by the creator/heads of the reboot: Dan Norton (art director), Shannon Eric Denton (the showrunner who spent ten years trying to get the project off the ground, only to see it last only a year on television), and the original Lion-O himself, Larry Kenney (who voiced Claudius in the new series).
The pair went into great detail on several plots that would have happened in Season 2: from the origin of Snarf, revealed to be an escaped experiment of Mumm-Ra, to a solo Slythe story showcasing his origins and hatred for the Cats (Lynx-o would have been revealed to have slaughtered the hatchlings of Slythe's tribe). This would build to him getting his revenge on the General and killing him, only to realize in the end just how enslaved his people have become to Mumm-Ra, resulting in Slythe giving the Soul Stone to Lion-O at the cost of his own life. They even talked about the controversal move of revealing Pumyra to be a double agent for Mumm-Ra, with the eventual resolution being her having so much power it warps her into a hideous being ultimately. After being beaten by Lion-O, who refuses to deliever the final blow, it's an all too willing Tigra who kills her. The action, of course, would lead to more sibling mistrust between those two.
Then talk shifted to the current Thundercat-lite program on Cartoon Network, Chima, and the ultimate reason why the show wasn't renewed: licensing and merchindising. The rebooted Thundercats show proved so costly due to licensing that creating an original program like Chima would prove more profitable. The true deathblow for the show came from toy sales; It and Young Justice were ratings successes in the 18-34 demographic and nobody was buying the toys.
It was interesting to note that, when the creators spoke of this, they remembered this glaring fact: ALL the shows that Cartoon Network broadcast on block at that time have been canceled. Two of the shows: Generator Rex and Ben 10: Alien Force hit their respective 65 episode limits (though in the former's case CN didn't air the final episodes, instead they were aired on iTunes), while other shows like already mentioned Young Justice and Symbionic Titan were canned due to lack of merchandise. Star Wars: The Clone Wars was ended, no doubt, so Disney didn't have to pay anything to Cartoon Network.