Developing character, as suggested in the post above, is a great way to get more mileage out of ideas you've already got in hand. It's also necessary, usually, if you want to get an audience invested in the plot line.
Here is one sort of specific idea: Create a back-story meant to develop a single character and to explain this character's motivation within the context of the story you already have written. In the back-story, you can create a new set of plot points, albiet on a smaller scale than what you've got in your 15,000 word first draft. Not only will this add pages to your book, but it will also add interest and investment in a character as well as lending a greater sense of reality to the book as a whole.
History, even made up history, makes things seem real. So...add some.
A final and larger piece of advice would be to plan on writing more drafts. If you always plan on following up a first draft with a second draft and then a third, you give yourself the time, opportunity, and freedom to develop new ideas over the course of these successive drafts. When writing fantasy, there will almost certainly always be plot points, ideas, and characters that can be better explained and developed. Planning to do multiple drafts means 1) you won't have to explain everything fully the first time around and 2) you will give yourself a chance to identify the points begging for development and 3) give yourself the best chance of writing something really great.