Well, Giant has stated that the point of this book is to deal with the conflict with the Linear Guild. It's unclear if Tarquin gets rolled into that or not, but it's certainly possible. Tarquin, Nale and Belkar seem like the most obvious possibilities, but with Malack already dead and Tarquin leading a massive army, it seems like it'd be cheap for Tarquin to bit the bullet anytime soon (I mean, he IS supposed to be a near-epic level adventurer and strong enough to best, or at least equal the OOTS in combat single-handedly).

So I'd say that other than "random character x" (like spiked devil) it's probably down to Nale or Belkar. Screw it, I'll stay with Belkar.