I always hated the AC and THAC0 system. Light and heavy armor basically operate the same
Back in the day I modded BG2 to give all armor damage reduction (%) and damage absorbtion (flat negation). I also normalized HP, so it didn't change much as you leveled up. Pretty much everyone started with 100 HP and ended up at 200 if they were very lucky and heavy on the CON.
It ended up a far more interesting gaming experience.