...If you mean Avatar as in "blue cat people," there's one coming. It probably won't take as long as the last one, though.
But if you mean Avatar the cartoon, there's a sequel series coming, involving the next Avatar after Aang. (I'm hoping that Aang is as crazy awesome as Bumi was in Last Airbender... or that Zuko turns into the new Iroh. <3 UNCLE IROH)
...But! For things that don't need sequels... Saw 3? 3 of those movies was enough for me; i haven't seen any since.
Another would be Firefly, actually. I know, strike me down with lightning and various blunt instruments, but a new Firefly series would never be as good as the original, I think. The reasons it is so popular are that it was awesome, and that it ended too soon, but if it started up again, it'd feel forced.