The Enigmo slug...Since it hasn't yet been resolved what it can really do, go wild.
Will Shane isn't deadHe was transported to a world below Slugterra. I base this on what happened during the ending of The New Kid Part 2.
Dr Black will find a new application of ghoulingNamely, he will ghoul humans. Imagine what could happen if a human is ghouled, or if it happens to molenoids, cave trolls, or any other living thing in Slugterra for that matter.
Fusion shotsWe all know that an Inkdevil and a Sonikin can produce some sort of sonic boom and an Infunus and a Tazerling can produce an electric fireball, but we don't know what all the other combinations can do. Go wild.
The Identity of the Twisted Man and his Little TwinThe King of Sling makes frequent allusions to one particular fight with the "Twisted Man and his Little Twin" that is implied to have concluded with one of his few bonafide victories. These two foes could possibly have been Twist and Loki. During the battle, Loki briefly transformed into a miniature version of Twist instead of creating a full-sized holographic clone due to a lack of power or experience. The defeat at the hands of one of the worst slingers on Slugterra possibly drove Twist to join Blakk, though if he was all ready in cahoots (perhaps not very used to Loki's new Ghoul powers), he may have kept mum about his loss.
Wil Shane is a prisoner of the Darkbain.It's pretty obvious by now that Blakk sent him to the Deep Caverns; what's less obvious is what he's been up to since then. The most likely answer is that at some point he was captured, which explains why he hasn't made his way back by now. He may even have been ghouled.
Doc will be Back for the FinaleKinda self explanatory really.