First, Ginny discovers that not only has Harry got her pregnant, but that she's going to have twins (the Weasleys must be genetically disposed to it). Then, Voldemort will take advantage of Harry's lack of skill at occlumency to project into his mind images of Ginny dying. Harry will be distraught by this, and will try to learn the secret of horcruxes in order to save her. He will, however, realise that the only person available to teach him this skill is Voldemort.
Meanwhile, the Order of the Phoenix (now led by Minerva McGonagall) will have created an army of inferi to try to aid in their fight against the death eaters. Unfortunately for them, Voldemort will know of a spell which will suddenly change the loyalty of all the Order's inferi, and they will proceed to kill all the unsuspecting good guys who they're out on missions with. The only survivor will be Flitwick, who will use his duellist's reflexes to survive, while he is out aiding a house elf resistance cell.
To make matters worse for the Order, while McGonagall will eventually track down and corner Voldemort, she'll be cut off mid-way through her avada kedavra by Harry, who will disarm her with a swift expeliarmus, explaining that there's a reason the unforgivable curses are called unforgivable. While the rest of us are face-palming over Harry's unbending moral absolutism, Voldemort will take the opportunity to kick McGonagall out of the window of the astronomy tower and to her death below. Distraught over what he's just done, Harry will decide "oh well, I guess I may as well become evil now after all" and go off and kill a bunch of innocent first years.
With the remnants of the Order in tatters, Ron Weasley and Fillius Flitwick will somehow manage to meet up. There they will decide that Ron should try to kill Harry, and Flitwick should go after Voldemort. There will be two hard battles. Flitwick will show that he isn't just a comedy little person, demonstrating a very acrobatic style of duelling, but ultimately will be unable to kill Voldemort.
Meanwhile, Ron will be getting pissed off at Harry, shouting, "You were the chosen one! You were meant to save us from Voldemort, not join him!" Eventually, though, he will manage to cut of Harry's legs with a slightly mis-aimed sectusempra spell, and leave him for dead. However, he will be rescued, and Voldemort will magically create a new pair of legs for him.
Finally, Ginny will give birth to her twins, and then die of a
Things end looking dark for the light side, but there is a chance that these twins may yet provide a new hope...