i would have liked more info in the epilogue too. however with that said, it would probably be too long if JKR had included all the details that we want.
as for harry's godson, harry would have been kind of young to be taking care of him so soon after remus and tonks died.
It had to be Ron and Hermione. If Rowling paired up Harry and Hermoine, there would have been no one to pair Ron with. I think the major purpose of the Ginny Weasley character was to provide a love interest for Harry.
I think the epilogue was brief because somewhere, in the corner of her mind, Rowling is considering writing about these characters again.