I posted this in the league forum where you asked the same question, but I will repost here:
Larry Doby (no one talks about him, and yet really, didn't he face all the same pressures that Jackie did?)
Josh Gibson (no one talks about him, and when you do hear about him it's a bunch of unsupported stats and myths...he was obviously great...would love to see a factual portrayal of his story)
Sandy Koufax (the dominance, the era, the high-profile nature of his religion (famously not pitching G1 of the 1965 World Series because it fell on Yom Kippur, Roger Angell referring to him as the WMEJB), the early retirement)
Christy Mathewson (a brilliant man playing at a time when he must have felt a little out of his element, the relationship between him and McGraw would make for a fascinating story)
Rube Waddell (hat tip to juice for suggesting this above... Waddell would never make it to the major leagues today because of all of his issues)
Pete Gray (duh)
Ted Williams (the dominance, the crazy upbringing, the contentious relationship with fans/press, the famous feats, the war heroism, the fishing, being re-embraced by everyone in his later years, the scene of the players mobbing him in the 1999 All Star Game still brings me chills...in the hands of the right actor and director that could be brilliant)