
I think what would happen would be that the movie would follow much the same plotline of the book until they reach Rivendell. At this point, Elrond tells Gandalf that Saruman is there too and they hold a council (basically a smaller version of the white council). Aragorn would be present at the council as well, seeing as he is trying to see Arwen secretly from her father and is therefore in Rivendell. It ends with them deciding (Saruman very unwillingly) that Dol Guldur must be attacked. They decide that they will send word to other important people to aid them in the assault (Radagast, Thranduil, Legolas etc).
Then the plot goes back mainly to the Hobbit and the Dwarves up until the edge of MIrkwood, every now and then switching back to Elrond, Aragorn and Arwen.
Gandalf says that he has important business in the south and goes to assault Dol Guldur with the elves of Mirkwood and the White Council. The battle (at the end of which Gandalf confronts the Necromancer and reveals it to be Sauron) along with the Bilbo saving the dwarves from the spiders would be the climax, at the end of which they realize that Bilbo is a capable member of the company and Gollum is seen climbing out into the daylight in search of the Ring.
In the second, it would begin with Gollum following the Orcs of the Misty Mountains toward the Lonely Mountain while they both search for Thorin's company.
The rest of the movie follows the plotline of The Hobbit, again occasionally checking up on Arwen. At Bilbo's confrontation with Smaug, the Ring could (in the slippery manner of the Ring) "fall off" Bilbo's finger, Smaug sees Bilbo, breathes fire, and in the heat the viewers see the engravings of the One Ring (although Bilbo doesn't).
In the Battle of Five Armies, of course ARAGORN AND LEGOLAS ARE THERE! (Those Orlando Bloom diehards will be happy) and blah blah blah, Gollum tries totake the ring from Bilbo during the battle, fails, and gets captured by those mean, tricksy orcses!
Sorry that was long-winded. I've though this through a lot.