You do me too much honor sir! How would I even go about proposing something like this to Bungie?

Hmmm...that's a good question. I would start with DeeJ since he's the community manager and send him a copy or link to what we've all read. Get his thoughts on it and see if it's something Bungie can work with you on. I honestly don't see a difference between what you're doing and what has occurred with the Dragon Age and Witcher franchises, as well as Game of Thrones series. Overall I think the more commitment you put into writing and developing your stories, the more Bungie will take an interest in, or notice of, your writing. At least this is what I believe. Thus far I have neither scene nor come across any published novels that complement or expand upon the lore of Destiny. However, this is precisely what you're doing with your writing, and I think once it becomes more polished and publishable it will be viewed by Bungie as an opportunity to enhance the world of Destiny. In any case, I definitely feel that once people who play Destiny start reading your work(s) they'll come to enjoy it, because--as another commenter pointed out--you bring the Titan and Ghost alive in your story.

You really have inspired me dude. Thank you for believing in me. I want to finish this project now more than ever.

Glad I could help :)