I'm possibly the only person in the world who thinks this, but...I wanted more Flash Forward! I was excited to find out what sequence of events was leading to 'The End', and I loved the way all the visions played out in the finale. Yeah the hero was the poor man's Jack Shephard, but he got the job done. (And that doesn't stop Person of Interest from staying on the air).
But, if it hadn't been cancelled after one season, it's likely the show would have gotten ridiculous by now.
Other, 'What if it had been cancelled' situations.
What if Lost had been canceled after season three? Abrams probably would have still left it with future Jack screaming 'We weren't supposed to leave', but it would have been assumed everybody got home directly following that episode and much more of the weirdness and mystery of the island would be open mysteries, and all the time travel stuff would never be part of the story.
What if The Simpsons had been canceled after the first few seasons? People would probably be begging for it to come back the same way they did with Futurama and Family Guy, and it'd probably be even more popular now.