Hayden Christensen and Jake Lloyd were, in my opinion, the worst things about I-III.
George Lucas took a chance on their acting skill and failed. Vader is one of the major reasons to see IV-VI. And the Anakin characters all but ruined my respect for him. Anakin in the Clone Wars animated series is portrayed so much better.
Jar Jar was... y'know, pretty bad. But that couldve been remedied with a few script changes. He was better in the sequels
now idk many other young actors from that era. But, as an above poster said, Leo would have been substantially more respectable. This is him in 1998:
JGL in 98/99
Edward Furlong in 97/98
Making his character 5 or 6 years older in TPM wouldve been a small price to pay for having respect for that character. And he couldve followed through with the sequels, Lucasarts was pretty good at getting entire casts to return.
So, I think that is the absolute least that would have needed to be changed in order for me to enjoy I-III. Believe it or not, I think a better actor in Vader's shoes in Episode III would have made that movie one of the best of the franchise. A couple other minor changes and it might have been as good as A New Hope.
Other smaller things I wouldve changed:
Young Boba's actor (same story as above)
Battle Droid and Gungan dialogue
Addition of some mention of Han Solo's origin
Give Grievous more to do or remove him entirely