JK Rowling has told what happens to the main characters --the full transcript of her recent session answering fan's questions is up on Mugglenet
I am pleased with Luna's future--she becomes a famous wizarding naturalist who eventually marries the grandson of Newt Scamander, author of 'Fantastic Beasts and Where to Find Them.'"
I couldn't see her ending up with Dean, despite their brief hand holding.
But Harry as Head of a Ministry Department seems a bit silly. He has no administrative talents at all, and should be a field worker. Although he is good (as the leader of the DA), at gathering people together to work for a common goal, so maybe it will work...