I haven't seen any patterns that particularly say they are for a
baseball jacket but after looking at pics of baseball jackets I would
think you could adapt a basic raglan or saddle shoulder cardigan
pattern. From the pics I looked at, the one feature all the baseball
jackets had in common was the team color striping in the ribbing
around the bottom, on the cuffs and at the neck. Some had the
sleeves in a different color to the body some didn't. So say if
the team colors you want to use are red and white you could do
the ribbing with the red and white striping, the body in red and the
sleeves in white or red depending on the look you prefer.
Here's some pics but I just realized they are not raglan sleeves but
might give you some ideas of how to adapt a basic cardy pattern:
Just an idea!