Idea vomit and feature-creep:
-Sprite damage. The nose, left/right eye and cheek could update with scuffs/swelling according to where the damage has been concentrated during the fight. Arm bruising too if blocks are being abused. This can be expanded to the torso with the addition of lows, or the jaw for uppers. It would be a nice visual cue for players to focus their defense and attack respectively. [The following ideas go together. Sorry for the length.] -Hold ducks / low punches. I've mentioned this one before. Admittedly it's tough to do with minimal inputs and touch compatible controls. I actually gave this a fair amount of thought, and wondered if instead of having a block button and duck button, you could instead have a standing block and a ducking block, and then have punches aimed at whichever zone you most recently blocked in. This removes the need to toggle stances, so quick ducks would still be possible. The duck button then becomes extremely versatile. Tap it once to dodge highs and change aim, hold it to block lows, let it go to start throwing lows, and when you wanna punch high again you can do a high block? IDK this one is really hard. -Context-sensitive aim. I don't like the idea of repeatedly whiffing highs when the opponent stays ducked. To combat duck spam, standing boxers could automatically punch down at ducked boxers after the initial dodge.
-Context-sensitive punch animations. What if there were different animations for the punches depending on stances and aim? For example instead of hooks, boxers throw straights aka crosses by default until the opponent is blocking, after which they throw hooks/uppers to try getting around the block. Or when throwing hooks downward at a ducked boxer, it could instead do a body-upper motion. This variety would create the illusion of more technical boxing without complicating actual gameplay. It would make guard breaks more believable too. The guard break animation could even change depending on what punch animation caused the break.