Adventures of Sonic the Hedgehog. I liked the 'silly' approach to the Sonic series, but the animation quality was horrible. I think they should bring it back, but give it a complete rehaul in the animation department. Make the wenviroments look similar to locations from the games and put the characters in their Sonic Adventure style designs.
I don't think they should have characters like Knuckles, Amy, Big, etc. in the limelight though. The one thing I loved about Adventures was how well the Sonic/Tails relationship was played out. Tails was actually a character that you liked, and not the annoying twit you see in the games and cartoons nowadays.
Knuckles and the others should just appear in cameos or season finales, but never be vital characters. The show should always revolve around Sonic & Tails.
I think most audiences today wouldn't recognize characters like Scratch, Grounder and Coconuts (they both appeared in Sonic 2) and so they should be replaced with Shadow, Rouge and Metal Sonic.
Invader ZIM: As much as i'd like to see ZIM return, the unaired season 2 was a bit of a mess and I think the show, if it continued, would've gone really downhill.
I didn't care much for the 'big' episodes Jhonen made, like Tak, Backseat Drivers, Frycook of Doom, and the planned Invader Dib movie. I dont like all the outer space nonsense. Just focus on ZIM being an idiotic alien who uses his technology to rip out eyeballs, steal organs, send pigs back in time, and mutate people into bologna, all while battling an insane child who has a another dimension stuck inside of his gigantic head.