As so often happens in planning work, I have come up with a few different versions of how things could work. I also took a couple of days off to let things sit, so I could come back with a somewhat fresher perspective and use that to see more clearly which route seems best.
I'm still not done, and I realize this work on the mythology and possible series is very important and must not be rushed, even though I am more than eager to move ahead with the next draft.
As I've looked over the options, I've also considered again that there may be a virtue in keeping different story ideas separate -- not linking them into a series. In general, in the fantasy genre, series are a good thing. Publishers like them, and fantasy readers love them. However, for a first novel, it is often best to have a stand-alone volume. Publishers are more comfortable with that from a first-timer. If that stand-alone volume could also later be used as the starting point for a series, so much the better, but it's not obligatory.
I will probably end up splitting the difference, doing the rewrite with an eye out to how it could fit into a larger series while also writing it in such a manner (mostly, ending it in such a manner) that is is satisfying on its own. I want that anyway from volumes in a series. There is nothing worse than ending an installment with a call that beckons "Sequel!" (I'll never forget my disappointment with the ending of Star Wars Episode V). The reader wants satsifaction each and every time, as well as the titiliation of more to come.
If I proceed with this idea of "stand-alone and/or first-of-a-series", then I should be ready to begin writing the next draft tonight or tomorrow night. I can always use the weeks to come to revise the ideas for the series. At least I do have a general framework now for the series, which I did not have before, and the key questions of how this first volume interfaces are nearly answered (they are answered, just in different versions, so I have to pin it down to which version I'll go with).
Anyhoo, it's been an eye-opener the past several days, seeing the potential for the series in various incarnations, three volumes, five volumes, and more. One thing has definitely stood out: I have other exciting stories I want to tell that are not part of this possible series, so whatever I'm doing with it, I need to get a move on -- there's so much more writing to do!