I've been going back and forth on this one, and I could see it going either way...For a Shadowcaster's shadow reservoir, can spells be heightened in the normal fashion (spending a spell slot of the appropriate level to cast the heightened version of the spell), or must the heightened version of the spell be specifically a part of the shadow reservoir (a la a sorcerer learning a spell at multiple levels)?
Right at this moment I'm leaning toward the former interpretation (shadow reservoir spells can be heightened by spending appropriate spell slots), but this might make it slightly more powerful than intended.
Any thoughts?
(Also I wonder about swapping spells out—retrainable, or are they fixed? So if a shadowcaster gets a heightened version, can they swap out the lower level version? This would be applicable to either interpretation, in the case of a spell one no longer wants in one's shadow reservoir.)