What kind of environment are your characters fighting in? You can add something akin to a pit of spikes that kills/inflicts damage, and players can try to push their opponents into those. Tai Chi characters' offensive attacks would do less damage, but assert a much larger backward force. Even if it ends up being a slightly weaker character, it would also be more fun to play because it always feels awesome to send one's opponent flying, regardless of actual damage dealt.
Another thing you could do is have a throw attack that doubles in power if used as a counter.
Keep in mind that the downside of using a keyboard/buttons as the control scheme, the player is very limited and is not going to get the actual "feel" of martial arts. You could rework the control scheme such that the mouse is used for special attacks, in which case the throw could be done by clicking on the opponent and dragging the mouse to indicate the force and direction of the throw. If the direction the mouse is dragged is the direction the opponent is moving in when the mouse is released, then force is increased greatly. This could be balanced by having the opponent be free to move until the attack is executed when the mouse is released, so successfully pulling it off would require skilled reflex/timing on the part of the player.
If you choose to add the mouse attack, it'd be best to used the WASD keys instead of the arrows because most players are right handed.