We can also discount Moelach, because we know he's in the Peaks. That leaves six.
Re-Shephir seems unlikely. She is much more of a skulker than one that would drive a normally peaceful people (I.E. I-4, Rysn, WoK) to go into an insane state where nothing is allowed.
Sja-Anat is also unlikely. She has been in Kholinar and in Emul, but not in Shinovar.
Chemoarish is one I think is in Shinovar. She isn't one of the three great mindless Unmade, so she could have the intelligence to stay in Shinovar and cause chaos. We don't know anything about her, so I'd say she's pretty likely.
Dai-Gontharnis is one I'm conflicted on. I'd love to see him, but I'd also love for there to be a creepy scene at the end (a la El) to show off this cool new villain.
Ashertmarn also seems likely. Shallan never actually defeated it, she just triggered the trap.
Finally, Yelig-Nar could be a really cool one for Szeth to instantly destroy with Nightblood.