I can't see a Dark Side supplement for two reasons.
A) Everything FFG has released thus far involving the Force has stuck to the rule that it's not what you can do with the Force that makes you good / evil, it's how you use it. If there is exclusively Dark Side or exclusively Light Side stuff, it takes away the behavior factor and regresses back to SAGA's stone age morality systems. Thus far, no career or Force Power is specific to light or dark, and I would hate it if FFG ruined that balance.
B) For people suggesting other eras, I'm still not sure that will ever happen. Take KotOR, for instance. All weapons (except those exclusive to a region of space) are generically labeled, so we don't need new gear. We have Battle Meditation, and most every other Force power you could want (I'm against system bloat). F&D's careers can reflect any Jedi or Dark Sider that we see in the games, and honestly, AoR can handle anyone in the Republic or Sith armies. If we got an era book, it would be a collection of starships and NPC statblocks, I think, which I find easy enough to re-brand, re-flavor, and re-spec.
Now what I could see would be a specialization or two, that, against my highest hopes, revolves around a single Force tradition, like the Matukai, Jensaarai, etc. I hope they don't, but those are more likely than a Dark Side book in my mind.
I want to add on to other eras a biit: I have a game that replays the kotor 1 events and mixes the old republic into it and the books already gave us all we needed there. The only thing that is excempted is the star ships of that era tehre was nothing in the books that really fit the dynamic light freighter. But else the books can be used already for that which would leave just noteable NPCs and fluff for era books. And seeing how FFG avoided that so far (statting noteable NPC vader as example) I take a huge guess that that is just too few things that could be put into an era book for them.
For a.) Don't overlook one thing there there is already a slight difference between light and dark side in some of the force powers. So that part of the argument doesn't really have a grip there. BUT that aside what would normally be in a darkside/lightside book? Taking other game lines into account: Force powers, talents, Fluff, new classes. If we look at what FFG made with force & destiny we see that:
1.) Force power are extremely generic thus you can simulate almost every force power you see form the films and most other game lines without even having to start houseruling
2.) Talents / classes: This COULD be something but probably more in a general addon (specs not professions btw). As we have seen there IS already a dark inspired jedi warrior spec. So there could be more which are light / dark inspired.
3.) Fluff / additional rules: If I take a look at the profession books so far I can see something there for force users as well but mostly profession / spec centered than light / dark.
In total: Light / dark I don't really see how they would do that as separate books. In one book: Maybe, BUT and here is the big but! All above things would also possibly fit into profession books. Thus it is way more likely that additional light/dark inspired specs would find their way into these books than into a separate light/dark book.