When PLee threw down the challenge, one of the first shows I thought of was Thundarr and how it needs to be brought back. The fact is, when thinking about a reboot, I personally wouldn't change much. The Alex Toth designs for the three main characters are perfect. I like the general style of animation they used and wouldn't want to deviate too much from that (like the 2002 He-Man did from it's 1982 predecessor).
With the reboot, I would want to fully explore the origins of the story and better trace how Earth got the way it was between the time of the meteor and the time of Thundarr. Some back story for the original series was written and a prequel movie was at one time rumored to be in the works. This never happened, but some story details were released. In essence, Thundarr and Ookla were prisoners and Princess Ariel gave up her status to rescue them. It seems like it would be a relatively simple story to set things up.
An element I would add is that this trio is not only wandering the devastated world to find humans and aid them against mutants, monsters and magicians, but they are on the run from a recurring villain who rules over much of what was once know as North America. Other villains from the original series would show up as well, including the only villain to appear more than once, Gemini.
It seems like there should be some goal in mind. Humans are oppressed and no longer rule the world. What caused the fall of humans? Where did magic come from? How can they ever defeat these powerful beings? Perhaps there is a "source" of all magic powers. Before the meteor, this source was sealed off from humans and only certain people in the history of time could access it. When the meteor devastated the world, the source was opened and tainted. During the series, Thundarr learns of the source and makes it his quest to seal it once again. This causes conflict between him and Ariel as she feels that she has proven that not all magicians are evil. As they grow closer to the source, Ariel's power grows, and so does her inclination for evil.
The finale sees Thundarr face off against Ariel. The final battle culminates in Thundarr using his Sun Sword to seal off the source (say that ten times fast). Along the way, the humans they have liberated began banding together and declare Thundarr their king. He turns it down and the three adventurers go riding off into the distance looking for more adventures.
From a behind the scenes perspective, I'd get Phil Noto on board to do some character designs. He's done some nice Thundarr drawings and seems to have an affinity for the character.