We are talking about Tyrion's mushrooms from ADwD, of course. Tyrion kept some of them after killing both Yezzan and Nurse, so there are still some mushrooms left to use later. And the fact that George deliberately told us that Tyrion kept them implies he already knows how Tyrion is going to use them later. He knows Tyrion is going to (have to) use that poison later. Back when I read I thought it might be used much later in the story on Illyrio and/or Varys as payback, but that would be pretty far down the road. Prior to that Tyrion is likely going to get into a lot of dangerous situations, so who could be the next victim?
Brown Ben Plumm or some other Second Sons? Crucial players in Meereen (Skahaz, Reznak, Hizdahr, the Green Grace)? Barristan Selmy? Victarion Greyjoy? Some Yunkish guys and/or Volantenes who are going to come?
Has anybody ever discussed that loose end?