Before any answers can be given, there are a few questions that must be asked.
1. Is the new book to be written for your current audience or are you going to write with the view to publishing?
2. If the answer is the former then your options are less controling in that you have a faithful following who will read just about anything you do. If on the other hand publishing is your aim then you have more constricting parameters in which you have to work. This of course is dependent on your own view of what is success on a personal and business level. Taking this further you have the following things to address.
a. Audience participation. Translated I mean how wide an age group are you going to target?
b. Content. General or adult content. Obviously the widest reading audience is with a general classification.
c. Appeal. How will the reader react to the content through the following criteria. ( Story line, believability, personal relationship, under lying message, continuity of story line, reality v fantasy, relation to real life.. etc)
d. Progression. Does the story lend itself to more than one adventure ie. more than one book, a series is more appealing from a publishing and commercial view point.
e.Market Appeal. How will the reading audience be drawn into the side products that the book would generate and what are they.
3. Do you as an author have the ability to work to a set time frame in a commercial environment and do you want to? The creative juices for want of a better description may not always flow when one is under pressure from strict time/production lines. To facilitate this problem it would be advisable to keep the story simple and contiguous which is what you have achieved with the Tarin Kael Series.
There are many more points which derive from just the few I've mentioned, however I will stop there and give you my own view of what I think is the appropriate direction to take

The Tarin Kael Series is commercial even now and I have no hesitation in saying that. Your audience as it stands is nothing as compared to a world wide reading audience and that statement should not detract from your achievements to date which I find truley remarkable. The downloads per chapter are approx 450 to 500 which is nothing with an audience of 2billion.
The fact that you have put the series on the internet should be used as a guide and a marketing tool, not as a reason not to publish it. The series will appeal to a wide reading base from 8 to 80yrs with just a few editing adjustments. This fact on its own puts it in the same category as The Lord of the Rings and Harry Potter both of which have the widest possible audience and have similar messages as does the Tarin Kael Series. My view is that you should expand on what you already have for it is shown by your ability to produce the chapters and story line that you enjoy writing it and this is the major ingredient to it being a commercial success. The series also leads to other opportunities which I won't elaborate on here.
What ever you decide I wish you well and thank you for the pleasure you have given me and my son ages 53 and 15. If you wish I am more than happy to help in other ways which I feel would be of value, if interested let me know.
There I've had my say shoot me down in flames folks

Regards,
RobJ