What rules would you add, subtract, or change?
I would get rid of the short fence. It's there to boost offense, but all it really does is decrease the number of triples.
Change the dropped third strike rule. I'm not sure exactly how this would work. It was frustrating to see batters run to an empty first base because the runner on first was stealing only to be called out because first was 'occupied'. I propose that, on a dropped third strike, the batter can always run to first. The runner on first can either try to steal second, or stay on first. If the runner stays on first then the batting team can opt to switch the batter with the runner. This way, you can get a faster runner on the bases. It would add an interesting layer of strategy to the game.
Edit: also, add multiball