The mission of the nobodies is not very clear. Since Nobodies don't have a leader or master, they choose their own fates. They currently pose no threat to The Wielders or the Heartless. Should the nobodies choose to side with either forces, it will be difficult for the other side to win. This does mean however, that once the nobodies choose a side, they will be considered a threat that needs to be dealt with.

Should the nobodies choose to fight for the Heartless, they will share a similar goal with the heartless, however, they require the defeat of the heartless to harness the power of Kingdom Hearts. (Of course once Xurturs has been defeated.)

If the Nobodies side with the wielders, they will share the goal of cleansing the universe of the darkness and to stop the Heartless from taking control of Kingdom Hearts.

There are a multitude of paths open to Nobodies, WHAT THE NOBODIES DO WILL HAVE A HUGE IMPACT ON HOW THE STORY COMES OUT TO BE. So keep that in mind, Nobodies play a very big role just like the other forces.