Necessary
Improve the Home Run Derby. Traditional format and multiplayer, definitely. Online would be sweet.
Online Roster updates.
Bullpen option in all modes.
Improve walks, allow beanballs.
Allow Alt. jerseys to be used in all modes.
Allow cloned Arrange teams to use the Alternate jerseys you buy for the main team.
Minor
More voices.
More batting/pitching stances.
More jerseys. For my White Sox specifically, their camo alternate:
and the '83 home throwback:
Good/Bad at home/away, and Good/Bad vs [team] as Special Abilities.
Really minor, but I'd like to see a cutscene where the manager comes out and takes the ball from the pitcher when he calls to the pen.
Pipe Dreams
I'd like to see a catcher mode. You could call the game, and also have the ability to go out the mound and talk to the pitcher when he's struggling, possibly improving his condition or making it worse.
Acquire the MILB license. Instead of having made up teams, actually go through the real AAA in Success/My Life modes. Allow us to play with minor league teams in exhibition.
Full roster editing capabilities.
Manager edit. Allow use to edit managers for every team on the main roster similar to how we edit lineups/rotation/roles for Arrange Teams so that the computer plays realistic. Difference is, we can also edit the look and attitude of the team's manager. In a bad game, you could have a cut scene where a hothead manager will storm out and argue, perhaps raising the teams' condition. Or he could come out and chew out the pitcher, and lower his condition. Or the manager could rally the team from the dugout for a boost. This part could be toggled off.
Dump Merluzzi, get a pro announcer, like Bob Uecker.