Well, I've got the Gods of the above mentioned race roughed out. Their true names can't be pronounced by human vocal cords at all and reptilian ones can only form a rough approximation so the humanoid races call those Gods collectively "The Lords of the Outside."  The "use" names of individuals in that pantheon are approximations, rendered phonetically by those who, having encountered them or their followers, are still sane enough to write down details.
Yeah, going a bit "Lovecraftian" with this group. The world I've created needs alien Gods who can go toe to toe with the pantheons we know and make those Gods swallow hard and know that their victory isn't assured.  The story demands deities that personify "evil" as we define it and take beyond anything any demon known to terrestrial man ever conceived of.....
That's right, not planning on making it easy on my characters come Book 4.

I thought of a piece of "groundwork" that I could put into to Book 3 to start setting up the situation in Book 4. I swear that it took longer to find the spot where I wanted to put it than it did to write it! "Fine detail" will need to be worked out -- and I'm STILL looking for a "girl next door" to "play" a pivotal role  -- but the broad brush strokes are being laid down for Book 4...

Consistency/Continuity

One of the biggest pitfalls in a continuing series is maintaining continuity and keeping things consistent and which are the hardest things to do. Even in worlds where magic abounds, there have to be rules and they have to apply in all cases. If, for any reason, the author needs for them to NOT apply, said author has to establish the conditions under which that happens. AND those conditions need to be established early on in the proceedings so that when The Hero/Heroine can't do something that they normally would be able to, the reader won't be going "NOW, wait a minute here!" and be totally thrown out of the story.  Do that to your readers often enough and your book won't go into a second printing and your publisher won't be interested in handling any more of your work!
A case in point is the Harry Potter series. Looking back over the series, it is obvious that J.K Rowling did not originally intend to write a multi-part series. The first Harry Potter book established the premise but had very few details about the world beyond Hogwarts and Harry's immediate environment. Which, for a stand alone book, is fine. As the Potter Phenomenon grew, it became necessary to add more details and for the first few books in the series, it went well. As the series wore on, however, it seemed to get harder to keep things straight and things already established were dropped by the wayside without explanation.
We saw the new movie yesterday and after we got home, something dawned on me.  Near the beginning of the movie, it is necessary to remove Harry from his Aunt and Uncle's abandoned home to a safe house. This evac was to be accomplished by a number of decoys magically disguised as Harry and their escorts all leaving at the same time, flying in the open air on brooms, beasts and various conveyances. This resulted in a nail biting scene in which the decoys and Harry were attacked by Harry's enemies.  The problem with this is that early on in the series, it was established that the wizarding world has a number of different ways to teleport that can be made reasonably secure. Now, I have to ask, given those ways and the importance of getting Harry where they needed him to be, why did they move Harry the way that they did, risking not only his life but their own?? Later on in the movie, when Harry and his friends were in danger, they simply teleported away and since the three of them had to join hands in order to end up in the same place, it was no less nail-biting.  
More annoying is that for the last few books/movies, things that should have been made known to the reader (if not necessarily to Harry and his friends) in the beginning were brought out of left field to serve the needs of the plot: the Sword of Gryffendor has the power to destroy the lockets that are the key to Voldemort's immortality, the three components of the "Deathly Hallows" [and WHY the hell were three ancient artifacts given a collective name that sounds like a location?] which have "something" to do with it all but the readers have no clue what.  These "revelations" might prove entertaining for most reading the story for the first time but are likely to be irritating during subsequent readings when it becomes obvious that the author needed something to complicate the plot -- as if the plot wasn't complicated enough!
At the other end of the spectrum is Tolkien's masterpiece "The Lord of the Rings." In the first book, "The Fellowship of the Ring" everything is laid out even if the reader doesn't realize it at the time and by the time the reader reaches the end of last book, everything has come together, fallen into place and makes sense.  Nothing needed to be brought in out of nowhere to make the plot work.
Keeping things consistent and the continuity straight isn't easy but if you are aiming to write something that will be read and enjoyed by your great-grandchildren's generation, then that is what you have to do.