I certainly hope you don’t make the hobo into the world’s saviour.

I like seeing movies about small, personal stories set against the backdrop of insane, world changing events (like Monsters).

You do need a clear antagonist though, and it also makes me think Aliens is a good basis for draw from. They very clearly created a “leader” for the Aliens, someone who Ripley had to beat in order for the story to be over (it works even better that she was a Queen protecting her own young). What if the leader of this group of reptilians was an outcast from his own society, in order to mirror your hobo’s plight?