I have nothing decisive to add to the many good suggestions that have already been made, so let me just add my support for a couple in particular:
- More variable personalities for both GM's and managers
- An ability for players to improve their fielding over time
- More options for choosing which data to savge and which to not have saved. No matter what the version, I find that I always have a lot of stuff to manually discard.
I have one particular hobby horse that I have been ridiing for years now, which is for independent minor leagues from which you can scout and sign players. This comes with a host of other things, such as a financial system for the minors, so I know it won't be easy. And also, I think the only other person who is really interested in taking this route is LGO, so maybe the lobby for this is pretty small!

But well, there it is . . . .