Yeah, I get RIOT is interested in making Elise more "thematic", but the reality is that Elise's kit is fine and her balance is in a good place right now.
Making a change like this to her Q has the potential to cause problems balance wise and game-play wise. Historically, RIOT has demonstrated a tendency to make changes to Elise when players haven't adapted yet to previous changes. So making a change like this to Elise's Q could very easily spawn a chain of patches from RIOT to re-balance her as it is almost certain that a change like this to her Q will take Elise out of balance.
This is the sort of change RIOT should've considered up front a couple months ago before making so many changes to balance Elise. Game play-wise the neurotoxin appears to be a poke mechanic that will give Elise a lot of consistent damage over time that she can spam onto enemies, so it would have the potential to shift Elise's game play away from the style of a spider hunting her prey to the style of a poke mage. So in the end, the change isn't even thematic.