Moving up and down in weight classes has been suggested numerous times. But I'm going to suggest it again.

In tandem with this suggestion, I'd also like to reccomend the possibility of cutting weight. You could cut up to two weight classes down. The drawback of cutting weight would be that you couldn't fight as often.
Cutting weight would be an option on the manage screen. To cut one weight class down you'd need a minimum of three fitness rather than two fitness to fight. To cut two weight classes down you'd need a minimum of four fitness rather than two fitness to fight.
A fighter's fitness and morale would take a hit from weight cutting. This would also make charisma more important.
In order to enable this feature, a fighter would have to be able to be ranked in more than one weight class.
Also, I'd like to see a pre-fight screen. The pre-fight screen would list a fighter's height, weight, and reach. As a supplement to this feature, the red and blue boxers would look different based on their respective heights, weights, and reach.
In the pre-fight screen you could also indicate an overall strategy such as win by points or go for K.O. or T.K.O.
As a minor suggestion, it's sort of annoying to have to allocate training points every time I hire a new fighter so it would be nice to have all twenty five training points distributed evenly and then make adjustments based on the fighter.
Also, I'd like to be able to shout things to my fighters during the round such as:
Knock him out!
You're ahead on points, ride this round out!
Go for the clinch!
Press him against the corner!
Also, as a way to make the game more difficult is to make the starting boxers stats lower. Your starting boxer can have stats that are quite high.
His stats should be based more on the kind of boxer that you can get with no reputation.