I think Nicholson as The Comedian would turn out just like Nicholson as the Joker. In other words he'd do an awesome job and the performance would be awesome but it also wouldn't really be The Comedian. I love Nicholson's Joker, but that isn't the Joker, that's Jack Nicholson playing Jack Nicholson pretending to dress up as the Joker. He's just chewing the scenery in that role. That's not a knock again Nicholson, it's just that back then people didn't respect comic book movies. It was Batman (1989) that changed all that, without Burton's Batman we wouldn't have all the comic book movies we have today. So if they could have gotten Watchman made in the 80s, I think it would be the same, they'd get Nicholson and let him go wild because people like that. It would be entertaining but it wouldn't really be the character. Although Watchman is one book that didn't need a movie. I really like it, but it's too unfriendly to adapt properly, especially as a movie. The movie that we did get wasn't as bad as I thought it would be. I even kind of liked it but all the nuance was lost, most of the character development was lost. Too many things had to be cut or changed. Not to mention that 2009 is a bit late to be making the movie, because Watchmen is a bit dated.
Yes, there were the Superman movies, but Superman is a cultural icon ingrained in the American conscious that goes far beyond comic books.
