Customizable team colors and logos
I would like more options along these lines, and it'd be totally dorky (and pointless!) but fun to have a template for creating uniforms, sort of like how EA games have that option in create-a-team. You'd be selecting from among certain letter and number fonts, jersey/pants styles and color schemes for each (some fonts/styles would have borders, etc.). This could be something extra we could do during the interminable Set Budget process. Other teams' uniforms could be seen under a tab on franchise cards.
Oh, and HISTORICAL BALLPARKS. This would add some new cities (Brooklyn!), while cities that have had multiple ballparks through their histories would have multiple representatives. They could use the same ballpark effects used in SLB. It'd be cool if each ballpark came with a different city logo that hinted at their historical nature, but I'd be satisfied with just the ballparks. Seems like an easy addition to me (except for the entirely new cities, which would require the assistance of a graphic designer).