According to an old article I read in a gaming mag (I forget which mag), Capcom was going to bring Astroboy to the Famicom but, there ended up being some sort of disagreement between CC and the owners of Astro so, they came up with Rockman (Megaman in the U.S.) instead.
That may just be a rumor but, it's really hard to say with all of the changes that are made to games and stories during the development stages.
EDIT: Air Zonk has power, sure but, Astro is extremely durable by comparison and his over-all strength is measured in horse power which numbers up to ridiculous amounts so, Astro would most likely be the winner of that one. I don't think that Mega Man could take him on either and MM is my all-time favorite game character.
Now here would be an interesting fight...
8-Man vs Zero