You don't choose a target creature at the time you cast Hypothesizzle. Rather, a second "reflexive" ability triggers when you discard a nonland card. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal. Notably, the triggered ability isn't a crime, but the reflexive triggered ability potentially is.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.