GlynG,
Wow! Lots to digest but I will throw out some information, concerns, and requests. First though great job on the beginning of your project. I am ALWAYS for printing a supplement ang believe that as long as it is well produced, people will buy them. You are off to a great start and if you can make good on these lofty goals you are in for a successful publication. I am pretty spread out on several different projects and tournaments, but I will assist wherever possible.
My number one suggestion is to find like minded people like Semajnollissor who are willing to dedicate some time and energy over the long run to this. You will find that your project will be much easier to maintain if you have several motivated people who can feed off each other's energy and ideas.
Remember that I didn't create
Raiders; I just organized it. FIFTY people created
Raiders. At the core of the effort was Corey (Necron Champ), Ben (Evil & Chaos), and I. Then we had Scarik and Xisor who helped me develop the Dark Eldar list. Then all the modelers, the artists, the scenario writers, the editors (thanks, CHROMA!!), and so on.
Also remember that
Raiders took about 15 months to create. My wife was quite perturbed with me for most of the time. We had people who backed out of their commitments, others that joined very late in the game, lots of arguments over deadlines and creative ideas alike. In the last two months there were two people doing 90% of the remaining work. Everyone else either ducked or just had nothing left to do. But enough about
Raiders...
My second suggestion is start tinkering with who might be going up against the Exodites (this is Epic after all). My first thought would be the Catachans (contact Nealhunt). Some Ork variant would be another. Find out who you WANT, then see if they are interested. Get a solid commitment from them because once you move onto the next step, a member of your team quitting can be disasterous. Remember the Army Champs involved don't necessarily have to do a lot of work, they just need to be solidly behind the effort and need to be consulted with. Regardless of their involvement, you want them to have as much time to prep things from their end: army list testing, modeling pics, etc.
Once you establish who is embroidled in this conflict, it is time to generate a story. Come to an agreement on the general plot, work up an outline, and make sure all the champs like it. Then start filling in the details. The story IMO is key because it helps keep the project focused. From the story you get a better feel for how the armies will come together, how the units interact, ideas for artwork
(key!!) and concepts for scenarios. How many different players are involved in this? Two? Three? Four? Are you going to be dealing with alliances? Three way conflicts? Is this an interplanetary war or a localized conflict? Armageddon's story had the IG and SMs working together against the Orks but there were also traitor Guard there as well. Swordwind had a staggered conflict and Raiders had a giant cluster-f going on as the Necrons intruded into a fight between the Dark Eldar and the Imperium. Whatever your storyline (and it should be no shocker that it will probably change eight times between now and completion) don't delay or ignore this area or you'll be sorry.
You may be tempted to just have this as a single army list. If you want it in print though, ask yourself how many people would want the book for just the Exodites? You'll quickly realize that other army lists are needed for this to make it to the press.
I think your modeling section will be INCREDIBLY important, but I really can't tell you anything that you don't already know. You've hit all the main restrictions upfront (all GW, no mixing IP, etc.). Just take a LOT of pictures - more than you think you will need. Take notes along the way.
I think that since you are going to have the Exodite modeling section, you may want to work on a Knight section as well for people who cannot get a hold of those models. If you make a commitment to show how to convert the entire army list, then you should feel comfortable putting the Exodites and the Knights in the same list. People who want to field the Knights outside of the Exodite list can always just draw it out of your list.
Psychogenetic manipulation: Genetic engineering sounds
so human.

Exodites may be the least technologically centered splinter of Eldar, but are still incredibly psychic creatures. So why not capitalize on that? The CW Eldar sing Wraithbone into existence from the Warp. The Dark Eldar have their dark occultic practices that allow them to do some gruesome but clever acts. The Exodites could have honed their skills into psychogenetic manipulation where dinosaurs could be bred to their design and trees could be slowly altered to bend and grow as they desire. You can also tie this into the World Spirit theme and into their control of the dinos. It may be that the Exodite who genetically manipulated the Dino has a distinctive tie to that creature that allows them operate as a team. It also keeps the need for the Knights since Dinos may be born wild and raw (thus require herding), and each one has to be manipulated from birth. This is all sounding somewhat Tolkien-esque but the Exodites are as close to Elves as you are going to get in the 40K universe so IMO that is okay.
Background meets the army list: I disagree that you should eliminate entirely the existence of already used Eldar units, although my suggestions are extremely limited.
I certainly see
Wraithgates still being a part of the Exodite lifestyle. There are simply not enough Exodites to be able to but up a substantial fighting force if their planet was to be invaded. I can easily see combat pacts being part of all Exodite worlds. Besides, Swordwind mentions the use of gates on Exodite worlds.
Rangers - Rangers fall under the Path of the Outcast as from Swordwind and it mentions them in the text:
The Path of the Outcast
Sometimes the rigid constraints of the Eldar path are
intolerable even for an Eldar to bear; such individuals
leave their craftworlds and become known as Outcasts.
Many Eldar spend years or decades as Outcasts before
they return to the Eldar path. Outcasts must bear the
terrible burden of their heightened Eldar
consciousness without the protection of the Eldar
path. Set free within the universe they are dangerously
vulnerable. Only Eldar of especially strong character
can survive for long as Outcasts. After years of
adventure and wandering, or sailing the seas of space
aboard the pirate fleets, most Eldar eventually return
to the sanctuary of the Eldar path.
There are many kinds and degrees of Outcast, such as
the wandering Rangers and the roguish Pirates. They
leave their craftworlds and live elsewhere, often
wandering the galaxy and visiting the worlds of men or
the Exodites. They are not welcome aboard craftworlds
except briefly, for their minds are dangerously
unbounded and attract predators from the psychic
realms of the Warp. Outcasts are also disruptive in
another sense, for their presence can distract the
young and inexperienced from the Eldar path by their
romantic tales of travel and freedom.