I'm tinkering with a few themes/ideas in the story: I'm not sure all will pan out; so I figured I'd toss them out here, for other's thoughts/discussion.
1) rivalary between teams(The cubbies and the Pittsburg pirates, in this case)
I thought this would spice things up a bit; give readers a 'bad guy' to boo but some of the events between the two teams as the season go may be a bit over the top. There's at least one on-field brawl and the one I'm not sure about: A all-star game diss to our star-rookie by the pirates manager. Standing back and looking at it: I think baseball's higher ups would call the owners of both teams and tell them to chill out: such open hostility hurts baseball rather then helps(Ah....but does it?)
2) late in the season, the cubbie's starting catcher gets injured. They have a back-up but even he will need a rest once or twice a week. They can't call up a minor-league catcher: they traded away their most promising catching prospects early in the season, to fill other weak-spots in the line up. So I figured, "Hey, I'll just have them decide to use my rookie-star as a backup catcher."
But a bit over the top, too. They're trying to make him into a center-fielder: to up and decide to have him fill in at a entirely new position; in the middle of a pennant race, takes some suspension of belief!
You don't want to make a hole bigger than you can fill. Mothys advice to budding authors.