For Dragon Ball, there really isn't anything I would change. The story had a nice progression and the characters were all well thought out and fit where they needed to. I would have only liked to see a bit more of young adult Goku prior to him and Chi Chi having Gohan. That is really my only complaint.
DBZ... I would have preferred Gohan to be a bigger deal after the Cell Saga. We all know how that went. I get why he was written the way he was, considering Goku had not been in his life and Chi Chi was smothering him but it could have been written a bit better. He ended up being more of a joke and I don't think that is what they intended to happen.
DBGT... Well, there was a lot of "filler" in this one. It felt like, at times, I wasn't even watching something related to Dragon Ball. Don't get me wrong, even with the bad parts I still enjoyed it but I feel like they could have done a better job with the pace and the writing. The first part felt like it was going in slow motion and after that, everything was sped up. The final fight of the series felt too rushed somehow. I don't know. I mean there is a lot you can change here that would have made it better.
DBS... I liked it. I still do. But they really need to slow things down with the power-ups. So for me, that would be the main thing I would change. Also, I would have added more to the universe 6 Saiyans. That whole part of the story felt too rushed.