Undoubtedly the version I most enjoy is the mixed breed. Whilst my concepts have him looking a bit too slick like Astro, and a bit too old (depending on the story development it may fit...), I still really like this version.
He feels incomplete, looks a little soulless. An idea I was considering was that he would be half robot half boy FO RL and that he may have a human brain and/or heart, possibly having memories or not feeling complete.
That may smell too strong of Astro though, as he was built by professor Tenma to replace the son he lost (in a car accident, I believe), but its just an idea. If I have a great hook for WHY he has human parts, then I will keep it going.
There is a Naruto nod as well, with the small badge appearing sewn in to a torso in the last jpeg... The broken torso is also story-specific... might not happen.