16Age Anakin, Eliminate Qui-Gon Jinn, and Make Obi-Wan the Seasoned Jedi
The acting antics of Jake Lloyd didn't do anyone favors. Child actors are very difficult to drag good performances out of, and it was a risk on Lucas' part that did not pay off with TPM. We get why he did it — to make this new trilogy more accessible
to a new generation of kids — but in the process, he needed to bring along his established audience for the ride, and Lloyd took us out of it.
The fault, of course, lies directly on Lucas' shoulders. Lloyd's character came across as whiny and stilted, and it's the job of the creator (writer/director) to create material good enough where that can't happen. What would have been wrong with aging Anakin to the point where we first meet Luke Skywalker in ANH? It certainly didn't keep the 1980s kids from connecting to the character and signing on for the ride. Picking a popular young actor of the day -- like, say, Smallville's Tom Welling, for argument's sake -- would have given us an Anakin who was ready-made for the adventure ahead and the epic creation of Darth Vader.
Also, while we love Liam Neeson, the Qui-Gon Jinn character would have been completely unnecessary with an older Anakin. We could have simply started with Obi-Wan Kenobi (still Ewan McGregor, because that was terrific casting) training the future Vader to become the badass we would eventually love, hate, then love again.