An LOE serves a couple of purposes. You fileis good enough in the eyes of WP that they want to encourage you to finish your file ( the sooner the better). Also, it enables them to offer you an overnight visit (which you should do).
To receive an LOA is not impossible, but improbable, unless something in your file changes that is really impressive, like a 36 ACT.
LOA's are not as plentiful as they have been in the past. They are mainly used for recruited athletes or other highly desirable candidates. That is why the LOE was invented. It's meant to encourage the candidate to complete their file and to offer an overnight visit.