I’m not sure if I’d do it at a convention that big; but as long as you know the characters, free-styling a TTRPG isn’t that hard. Basically, you just need to know who the PCs are, who the major antagonist is, and the latter’s intentions. If you know intentions, you don’t need to know plot; it’s whatever happens when the intentions of the antagonist bump up against the PCs. At that point, they’ll tell you what they do, and the antagonist reacts to that, and then they react, and pretty soon it’s time for the big fight.
‘Course, you don’t end up with much left over for the next session, but hey: it’s a con game. There ain’t gonna be a next session. So front-load that puppy.