When Earth is threatened by a cosmic war, a young woman with newly-awakened powers becomes mankind's only defense.

Isn't this just 'Dark City' with a female protagonist?

I wish I could offer better suggestions. At least for me, I'd love to follow her as she awakens to her powers. It must be thrilling and terrifying. Maybe with no mention (or a vague hint) of a greater cosmic war?

You got me there TimmyElliot.

I think the originality will come when I actually construct the mythology that goes behind it. I have no idea how to put it in the actual logline without distancing the agent or manager from it, but I also don't want the story to be too familiar, which seems to be a tough line to straddle within the apocalyptic genre.

What if I changed cosmic war to an interstellar, holy war with gods warring over who gets to reclaim the earth for his or her self? I want her powers to be spiritual in nature anyway and the story to be a question of what if God is a group of alien beings.

As a Lovecraft/cosmic horror fan, you won me with this talk of "ancient gods." Now I am hoping the young woman dies miserably, earth is destroyed and Yog-Sothoth reigns!

Actually, I am liking this revision you did better. But, maybe that's the problem. This whole cosmic war, just feels like too much that has already been done, from cthulhu mythos (where it turns out badly for humanity), to, say, many x-men/fantastic four story arcs (where it turns relatively well for humanity). Actually, isn't the story about a cosmic war, with ancient gods and humans in the middle, the basis for nearly all our mythology? What's so special with this story?

It sounds like it could be a logline for any of the X-Men and the Fantastic Four Cosmic War story arcs (such as the Kree-Skrull War).

Not seeing anything special yet.

Or on second thought:
When Earth is threatened by a cosmic war, an independent, young woman with newly-awakened powers is mankind's only defense against warring, ancient gods.

First, thank you for the critique!

I want to keep her powers a bit of a mystery that finally become fully realized at the Third Act Plot Reversal.

How's this:
When Earth is threatened by a cosmic war, an independent, young woman with newly-awakened powers is mankind's only defense against the return of ancient gods.

Overly vague and cryptic. The purpose of the logline is to encourage a request for the script so as to read it. This logline says to me "Who cares?" Tell me why and how these things are done, and if I'm hooked, I'll read it.

Oh...and do it under thirty words. I have a plane to catch. ;)

I like your logline. Only comment I would have might be describe a trait of the "young" woman. Her youth may not be relevant but moreso the struggle with the new powers. Otherwise, I like the rest of the logline.

It's strong, but:

What is the woman's occupation/strength/flaw?
What are her powers?
And who is warring with Earth?

I like it! My only concern is that "cosmic war" conflicts a bit with "only defense." Meaning, I see no "war" since we would be annihilated, which may be what is certain to happen, no? So, maybe When earth is faced with certain annihilation from (whatever may be), a young...