For a theremin, you might consider a Whammy-like pitch preset through the Synthonizer. Not sure how that would track … With access to an expression pedal, it might be good to run through all the pitch-based presets solo, and note how they track, crossfade, and/or glitch.
The ‘pitch-shifting’ Delay algorithms (like Tape Echo) give unusual effects when adjusted, as do some of the Reverb algos with SIZE. When I think ‘spooky’, it’s hard to avoid the BlackHole algorithm. Plenty of multi-FX to fine-tune there. Heavy modulation will bend the apparent pitch in freaky directions.
Off the top of my head, here are a few preset links that may / may not help out here:
Klang Mangle Organ
Rubberband GlitchShow More...
Poly Expressive 2