I have to wonder if the complaint of your story being too short is more a matter of perspective as opposed to something that needs to be 'fixed'. I admit, I thought the story felt short as well but then I question: is it short because it actually *is* short or that I enjoyed it so much it *seemed* short? I'm leaning toward the latter.
I also think it would be awesome to see a continuation of this story into an extended and cohesive VN of say... 10? chapters? (Arbitrary number.) It would be a full-on D&D campaign. ;) I certainly wouldn't complain. :D Besides, you put so much effort into the creation of these characters that it would be a shame to let them languish on the laurels of this one adventure.