I think a movie about the 1998 home run record chase by McGwire and Sosa would be a good start to a movie -- perhaps the first 15 minutes are a really fun, lighthearted look at that summer, before the other shoe drops and the steroid scandal takes place years later and the Mitchell report is released. It could be done in Fincher's Zodiac style, a "you're there" journalistic approach, since there is so much ground to cover.
I think a Cal Ripken biopic could be done well if it was framed around the 1994 strike and his streak to break the record. I envision something wholesome like The Rookie or Invincible.
A Roberto Clemente bio-pic seems ripe for the taking.
I also think a Steve Bartman film would be good, but I feel so bad for the guy that I almost don't want it made to bring more attention to him.
I never had friends later on like the ones I had when I was twelve. Jesus, does anyone?