Well I just got this game and it is GREAT. By far the best boxing sim out there but I do have a few suggestions. I have only been playing this game a short while, so some of these things may occur and I just havn't seen them yet.
More purchasable items. Perhaps skill specific training items. (i.e. heavy bag, speed bag, sway bag, treadmill)
I'd like to see cornermen and cut men added. You would have to hire these and they'd take a cut of every purse. The corner man would be in charge of strategy. A better corner man and your fighter would follow your suggested round strategy more closely. The cut man would treat cuts and swelling between rounds. A better cut man and there would be less of a chance of the fight ending due to stoppage.
Being able to buy a larger "gym" which would then allow you to train more than 6 fighters. Perhaps 2 upgrades each adding 2 fighters to your stable. These would of course have a large purchasing price and then charge a monthly rate as well.
The ability to hold sparring matches between fighters in your stable. These would raise both fighters ability, but have a high injury rate.
When renewing contracts, you should be able to choose the terms of the contract.
Other managers could try to steal fighters from you. You would then be notified and you would have to make a counter offer in order to keep the fighter. Also, fighters with low morale would be less likely to stay with you regardless of how good of an offer you make them. Also, if a fighter has a low morale and/or loses several matches in a row, he should be able to fire you.
When challenging a fighter, they should be allowed to reject the offer, like when challenging a fighter that is like 7-10 rankings below you, or is clearly outmatched. Other managers should be looking out for their boxers too.
Also, other fighters should challenge you and you have the option to reject or accept the offer.
When challenging a fighter, you should be able to negotiate the purse splitting with the other manager. Usually a larger split of the total purse would go to the higher ranked boxer, but your negotiating skill would tie in to this.
There should be different fight venues and these should be determined by a fighters rank. Like fighters below rank 75 would fight in local gyms and top 10 fighters would fight in Vegas or at Madison Square Garden.
When you have a fighter ranked in the top 10 you should be able to negotiate Pay Per View contracts. An example of this would be like a 6 month contract in which you have to fight at least 2 top 10 matches. If you fail to meet the requirements, you'd have to pay some sort of penalty or perhaps your manager rating would go down.
Endorsments should have some sort of requirements. Maybe bonuses if you meet certain conditions. Like scoring two knockouts during the contract, fighting a certain number of fights, winning a certain percntage of your fights, or going undefeated.
Fighters should have a charisma/fame rating. High charisma fighters would draw bigger purses for their fights and fighters with a high KO rate would also draw bigger purses.
There should be a couple more options for fight strategies, such as Go for KO. If your boxer is way behind on points in the later rounds of a fight, you should have the option to go all out for a KO. He would totally disregard defense, thus leaving himself vulnerable to being knocked out as well. Also a Protect Cut/Injury option. If your fighter gets injured during a fight he could focus on defending that part of his body to prevent the fight from getting stopped. This would be different from the Defensive option in that your fighter could not regain any energy during the round.
That seems to be all I can think of for now. Back to playing and thanks for listening. :D