1) As it is Cameron Crowe wrote and directed it and was one of 4 producers. They could've fired Crowe before shooting started, gotten Robert Towne and some other rewrite experts in there and completely re-written the whole thing; or better yet, just used an entirely different script altogether. For Jesus' sake, don't make every character sound like Cameron Crowe!
2) It could have been Avery (Kelly Preston), Jerry's finacee briefly at the start of the movie, who had the nervous breakdown and then the "vision" about sports agents being... something, instead of what they are. Avery could've broken up with Jerry (in the film it's the other way around), joined the agency, then quit very soon. Jerry could remain an agent like agents are in real life, and the film could be a realistic portrayal of a sports management company, neither idealizing nor unrealistically condemning the industry, with occasional shots of Avery on her Crusading quest as comic relief.
2a) This raises the interesting question of whether Dorothy (Renée Zellweger), who in the film leaves the management company with Jerry and later develops a romantic relationship with him, will leave with Avery, and still eventually develop that relationship with Jerry. This would give Dorothy's character much more to do than in the film as is, where pretty much all she does is wait around for Jerry to sweep her further off of her feet.
3) The scene where Rod (Cuba Gooding Jr), an NFL wide receiver and the only one of Jerry's clients who leaves the company with him, gets up on his feet after an apparent injury in a "Monday Night Football" game, and dances all over the place in front of a wildly cheering crowd, proving that he finally understands what Jerry (or Avery in the alternate version) has been telling him all along? No. Just no.
4) Get that guitar away from Frank "Cush" Cushman (Jerry O'Connell), another one of Jerry's clients, who wavers for a while but decides to stay at the agency instead of staying with Jerry, and keep it far away from him as long as the cameras are rolling.
5) Include a long angry rant by someone directly into the face of Jerry or Avery or whoever it is who has written that manifesto about "quality" in sports management. The angry rant is about how the job of a sports agent is not about "quality," but quantity: the number of dollars, Euros, pounds, Yen or what have you, which the client receives. The angry ranter could point out that if the manifesto writer is really interested in quality, there are a lot of volunteer positions open in the world's disaster zones. The ranter will definitely denounce American collegiate athletics as possibly the most monumental and shameless rip-off of athletes in the history of the world so far. You could give that rant to Sugar (Jay Mohr), Jerry's protégé who stays at the firm, is selected by management to fire Jerry, and systematically takes all of Jerry's clients except Rod. The rant would give Sugar's character more complexity, making him less of a one-dimensional weasel. (Of course, this and a lot of my other suggestions assume that you agree with me that Jerry's manifesto is ridiculous. If you agree, then you've just about got to take suggestion 1 at the least.)
6) Jerry's relationship with Ray (Jonathan Lipnicki), Dorothy's son, would be much more interesting if Ray started out hating Jerry. Give Jerry's character more work to do. Or,
6a) Eliminate Ray and a few other characters. There are too many characters all trying to be special for one movie. Pick yr battles.
7) Another long angry rant, this one about the stupidity, hypocrisy, brutality and infantilism of sports, and about how athletes, coaches, sports journalists, team owners, cheerleaders, super-fans, etc, etc, are generally every bit as empty and horrible as agents, although agents are much less likely to be revered, for some reason. This rant could also be delivered by Sugar, or someone else.
8) NO BONNIE HUNT! If you don't eliminate the character (Laurel, Dorothy's sister), then at least re-cast the role. It's interesting: I was just on Roger Ebert's website, researching, trying to comprehend just why so many people mistakenly think this movie is any good. Ebert called Hunt "almost always a delight to watch." I've never been the slightest bit delighted to watch Bonnie Hunt, and I'm right and Ebert was wrong.
9) In the movie as is, Jerry and Rod supposedly both become much more deep and less concerned about money; and as a result of this, of course, they end up striking it rich. Shit or get off of the pot; either stop preaching against a concern with money (or keep that nonsensical preaching in the mouths of comic relief like Avery if it's Avery who has the breakdown, writes the manifesto and strikes off on her own), or show the characters actually having to live on much less money, but still being happier. For instance, maybe Rod has a career-ending injury before he gets that big payday; and then later we see him as a high-school football coach, or maybe in some other modestly-paying job: and he's happier. (My feeling is that more money definitely will make you happier, and will until we pivot to that Gene Roddenberry-style post-economic world; and that anyone who tells you different is either poor and in denial, or rich and so emotionally fucked-up that not even money helped.)
10) Let Jerry and Dorothy be much more flawed. Because people are deeply flawed. Always. Cuba Gooding was in a very good movie about the same time he was in Jerry Maguire: James L Brooks' As Good As It Gets, a film in which characters have horrible personal problems, and don't solve those horrible problems, but still achieve some significant triumphs and improve their lives. In other words: A realistic cheesy mainstream feel-good movie about rich white people.