I love tolkien but I'm not an expert, so take my words with a grain of salt; all of this is just speculation.
Everything in the hobbit happened because of gandalf. He lead the dwarves into killing the dragon, so the orcs moved into attacking erebor, and the resulting war cemented the alliance between the three races. All of this while he was driving sauron out of dol guldur, and acquiring other small advantages (like killing the goblin king, befriending Beorn, recovering orcrist and glamdring, and so on).
So, what would have happened if gandalf wasn't there?
First of all, the ring would eventually have returned to his lord's hand. Sauron himself could have extended his dominion all over mirkwood. The free people probably would have slowly estranged one to another, to the point that they would have never fought together against the common enemy. Dale would have never been rebuilt by its one true king, nor could have been a new king under the mountain; during the upcoming war of the ring, sauron would have had an easy time with a dragon on his side and no one defending that zone. He could have burned mirkwood to the ground, killed aragorn in a blink and then annihilated rivendell... Eventually, he would have proceeded in crushing between anvil and hammer lothlorien, rohan and gondor.
One could extract a cool front out of this, but it won't be tolkienesque. Tolkien is full of deus ex machina and arbitrary coincidences. Something totally incompatable with dungeon world.