Sometimes a really great story just doesn't get a chance to conclude on TV, and there's nothing anyone can do about it but wonder what might have happened. For me it's Stargate Atlantis. The showrunners had plotted out a final season, and when that didn't happen they wrote a movie which was cancelled also. Years later I still sometimes hope they'll announce some sort of final movie or miniseries but I know the ship has probably sailed at this point. What other shows do people still want closure for?
Freaks and Geeks goddammit. Is four seasons an unreasonable request? I'm the kind of nerd who listens to DVD commentary and what they had planned is full of unrealized potential. The actor who played Sam had a growth spurt while they were waiting to find out if they were cancelled so they were gonna put him on the basketball team, driving a wedge between him and the other geek. Daniel was going to start hanging out with the geeks because he liked playing D&D with them but he was going to remain mostly with his clique. Lindsey was going to become more of a stoner after her time following the Dead, start dating Daniel and that wouldn't have gone over well with Kim and Nick.
At least Judd Apatow had the last laugh being a comedy writing and producing golden boy now. And Paul Feig is directing the new Ghostbusters. Pretty much everyone went on to have a career afterwards. But that last episode is heartbreaking, not just because that was the nature of the show, because you'll never see those characters again.