That's an interesting question! What could have been done to the second installment of The Hobbit to make it a more rounded experience as a single film? Within the confines of the original book, probably nothing, that ending is a fairly natural one and comes at the end of the third installment. But they might have found a logical conclusion in some of the material not found in the book. Like--
SPOILERS AHEAD if you haven't seen it
the totally invented and IMO unlikely romance between dwarf and elf maiden. Their story could have concluded for good or ill and if that relationship had been given a little more air time, maybe it would've stood as a decent closer. Actually, I didn't think much of that added element, I guess someone high up just read the script and said 'Where's the romance? Gotta have a romance!' A better ending may have been to center on Gandalf's solo struggle. If my memory's right, that is all extrapolated from a very few lines in the book where Gandalf leaves the group and when he comes back he says he had a run in with The Necromancer, but the extrapolation seemed legit to me with suitably epic special effects. They could have ended the movie going back and forth between the dragon battle and Gandalf's battle. The intervals between the back and forth getting shorter and shorter until Smaug breaks free of the mountain flying to Lake Town bent on vengeance and a close up of Gandalf with a belated realization and a single spoken word: Sauron.
But, basically I agree with you, Terry, the ending seemed abrupt. Maybe the flow is much better when viewing all three and Jackson is already dreaming of moving all those boxed sets?