All things being equal, if IE were moved to 3rd level and became effectively a ribbon (given the 7 day prep time required), what would people propose to replace it at 9th level - at least thematically, if not a fully fleshed out ability?

I think something poison related would fit. Maybe something along the lines of being able to obtain poisons at a reduced cost (50% redux would be fine), ability to change the delivery method [contact, ingested, injected] (I'd say during a short rest, but not adamant about it), and maybe something like adding poison use to their cunning action suite. Coating blades and arrow heads, that kind of thing.