One of the AD&D systems that we have always taken issue with is the THAC0 table and in our game we have gotten rid of it completely. A character's THAC0 score is determined by their speed and armor. Our opinion is that armor has nothing to do with a characters ability to avoid hits in combat and should be taken out of the equation
Instead of a THAC0 table system for hit determination we have begun using a dynamic ToHit system that pits a characters skill and levels against that of his opponent. This system was first outlined by Dave Medvitz in his NET ADVANCED DUNGEONS & DRAGONS PLAYERS SUPPLEMENT. As you will see, we have taken parts of his system and replaced others with our own ideas to come up with a solid system.
How it works
The player winning the initiative roll would use the first action phase to attack and the player losing initiative would use the first action phase to defend against that attack. Once the first action phase has been concluded the second phase begins.
If the attacker has an additional attack they can now make that roll. The fact that the initiative winner is attacking forces the defender to make some type of defensive maneuver thus using their phase 2 action. It is possible for a defender to refuse the defense roll in order to do something else but they must understand that the attacker, facing no defense, will almost surely hit and being hit while trying to perform any other action should severely limit the possibility of success.
In the last combat phase (phase 3) the attacker (or initiative winner) would have and opportunity to make another move. If they have an attack remaining they can use it, if not, they can use the action phase to draw another weapon, move, use a non-weapon proficiency, etc. Likewise, if the defender finds that the attacker is out of attacks and they have any remaining they can use the last action phase to launch a counter attack. If this happens the initiative winner would be forced to drop whatever plans they had for the 3rd action phase and defend against the attack. The only possible exception would be if they were able to use their 3rd action phase to get out of attack range.
At the close of the 3rd action phase we roll initiative and the process begins all over again.