the point of the series is that being a grown up is about maturity, not age, that while everyone has to grow up you can't force maturity and its okay to just be a kid while you're still a kid. the 'too good for this kid stuff' Naota is contrasted with his manchild father who never matured and the bushy brow guy who is the kind of man Naota will grow into if he keeps trying to fake maturity.
The climax is Naota setting aside all pretense and just telling the flaky alien girl he has a crush on her. She in turn finally makes a responsible decision in her life and doesn't drag the impressionable kid off to space just because he amuses her.
You could possibly argue that Atomsk, the godlike power everyone craves, is symbolic of the power of authority wielded by successful adults which childish people are envious of yet don't understand, but it could just as easily be a mcguffin and nothing more.
and the final answer is that it doesn't need a sequel.