18. If you could change one thing about canon, what would it be?
Well. On the shipping front I suppose I could wish for a H/Hr resolution to the series, say. On the other hand I really wouldn't waste too much effort trying to induce Rowling to write such an ending; her OBHWF 'romances' were so simplistic and uninspiring I don't think she could/would write anything better for Harry and Hermione.
HBP was a bad book, and DH was a total disaster, a complete embarrassment for Rowling and catastrophe for her series, so I guess I could wish for a change there. But that would be a HUGE wish, to fix all of the garbage in those last two books ... I'd need a score of genies to do the job.
So I'll be clever and wish for a much smaller, more reasonable change ... but one which might still result in a quality ending to the series.
I wish that J. Rowling, on either realising that she was unable to properly finish what she had started - or on being persuaded of same by her editors - had sought assistance in plotting and writing the last two books of the Harry Potter series.
I guess that's not really in the spirit of the question - changing 'one thing about the canon' - but it's one change in the real world, which is close enough. :-)