After reading some of the top comments, I've come up with a formula which would ensure my purchase. It's based on Shannon Briggs.
I want Briggs available as the guy who pushes you in career mode. What's that, 9 unread emails? Lemme go read them. Look at that. They've all got attachments. Each one is a video of a shirtless, board shorts clad Shannon Briggs, doing the best he can to will me to the top. "Let's Go Champ!"
Briggs will be available in the game, but only in a special way. In a career mode, he will fight you but only when you're at the bottom of the ranks and the game has determined you're terrible with the sticks. That's when you get to face Briggs, and he bears the shot out of you.
Online, Briggs isn't immediately selectable as a fighter. However, you can select him as an audience member for big fights. Basically, you're your own online party, sitting in for fights involving whomever holds an online belt. During the fight, you can talk shit to both fighters. The only way you get to challenge the winner, as Briggs, is if you've taunted the winner so effectively that he offers you a challenge. There one more stipulation......
As mentioned in previous comments, a wagering system seems like it would go over well. Something like career earnings. Briggs only accepts the challenge of the Champ is willing to wager it all.
Aside from that stuff, here's what I'd like to see:
Fighters need to have their own style. It needs to go beyond the way they throw a hook, or their clothing. Somehow, make intangibles tangible. This is especially true when you're fighting the cpu. Just because it looks like I'm fighting Holyfield doesn't mean it feels like I'm fighting Holyfield.
Get more boxers in the game. Seriously, so many legends and currents get left out. Even the last gem game systems had virtually unlimited storage. Let's use it to add more fighters. Shit, even make the addition of fighters interactive. Let us vote on who gets created next.
Like to make money on dlc? That's fine. But don't let me buy packs n points for attributes. I don't want to pay to be better than other players. Let me buy more obscure boxers. Take the time to make a really good, realistic, Jimmy Thunder or Gerry Cooney.
One Punch Knockouts Need To Have Some Kind Of Really Unique And Understandable Formula OR REMOVE THRM COMPLETELY. For a knockout to seem realistic, I'd better be completely over matched, have stopped defending, or taken some kind of huge gamble, at the same time as my opponent.
Blood. Bruises. Injuries. Their needs to be lots more variations of blood and bruises, as well as their consequences. Really bad cut over my eye? I want the red to warn about stopping the fight. I want to lose vision on that side. Let me wipe it away to regain full vision. Spamming a punch, or did something to injure myself? Ok. Now my right hand is broken. Let that happen and let their be consequences.
Knockdown Animations. EVERY release has had the cheesiest animations for knockdowns. Also, there are so few of them. The game gets redundant after only a few hours of play.
More serious consequences for quitters.
TALE OF THE TAPE: (online) A way to leave feedback about fighters which gets compiled then made available for reading while the fight loads. About to fight a guy who only counters, I want to know. Fighting a guy who likes to brawl, let me know. Some how. Some way. Make us feel unique. The more I play, the more data there is of me. 2k had this in 2k5 football. It made a profile which charted tendencies. That would be awesome for boxing games.