They've said in the past that there will be mechanical repercussions to using the dark side, I'd imagine that would also include more concrete rules on when one becomes a darksider, or at least more on what that really means in game terms.
It's in the core rulebook that such a thing is on the horizon. Page 278 for instance, has this tidbit...
"Full-fledged Jedi and other powerful Force-users suffer unique penalties for doing this* that are not detailed here."
*converting Dark Side pips on the Force die to usable Force Points by flipping a Destiny Point and suffering strain..
While I doubt that FFG is going to give much in the way of full-blown support to Dark Side Force-users as PCs (much as how the Age of Rebellion book doesn't give much support to Imperial-centric campaigns), there's bound to be some sort of "dark side tracking/penalty" mechanic when FFG pulls back the curtain on playing "full-fledged Jedi and other powerful Force-users" since the ability to get a Force Rating higher than 3 will become officially available.
Lorne, I'd be surprised if said "dark side tracker/penalty" mechanic doesn't have more of a narrative base, particularly in comparison to the OCR/RCR d20 games (which could involve stat penalties at the higher end of things). Personally I'm a fan of the idea of a "threshold" that once the PC crosses it, they're "consumed by the dark side" and become an NPC (the approach used by d6 and Saga Edition). It could even tie into F&D's version of the Obligation/Duty mechanic, with a positive value being thing akin to AoR's Duty, while relying on the Dark Side results in the value going down, thus being less likely to trigger.