on a more grounded note, I find it pertinent to think back on the role 8-balls actually played in Vriska’s life:

You consult with these ominous globes, but routinely destroy them in frustration over the PUZZLING GUARANTEED INACCURACY of their predictions […] and with each you destroy, you add to an insurmountable stockpile of TERRIBLE LUCK. You have to stop. But addiction is a powerful thing.

the time when Vriska was actually “using” 8-balls was her lowest point; when even her signature luck had abandoned her. given Scratch’s warning that Vriska’s Light “will abandon [her] once again, perhaps for good”, then, the fact that an 8-ball acts as the symbol for the final chapter of her journey is more than a little ominous!

in addition to being metonyms for Mindfang and Doc Scratch as characters, the ancestral journal and the cueball are both objects that had their own very literal influence on Vriska’s life in and of themselves; so it seems reasonable to expect that the chapter marked by an 8-ball, in addition to whatever confrontation it may involve, will also shine a spotlight on Vriska’s addictive personality in order that she might break free from that cycle of addiction (…or else regress in some seriously reality-shaking way.)