When a downtrodden street-dweller uncovers an underground humanoid-reptilian race’s conspiracy to enslave humanity, he risks his own life to protect that of an innocent little boy.

That's great food for thought.

I pictured the reptilian leader as a huge, mythical snake that's embedded in the deepest chamber of the sewer system - one that's so grotesque and nasty it strikes at its human sacrificial offerings and bites their heads clean off.

This everyday hobo would have to overcome his fear of snakes to kill it.

This idea is much less well-drawn than your own (mirroring the hobo's plight is really great), but seeing an oversized snake bite a human's head off in one go just seems so poetic.

I think I actually stole that from one of Lui Kang's fatalities in Mortal Kombat.

Come to think of it...Reptile is straight out of the MK Universe too. I'm so busted.

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?

>> as no caring parent would generally let their four or five year old hang out with a hobo

Make the parent(s) tweakers (meth addicts). They'll get uncaring fast.