What I love most about BG1 is battles in open areas: if you can't just straight up tank the enemy, you can navigate around the area, which adds complexity to the battle. The most memorable battles occur when you are forced to move a character, and then, unfortunately uncovers another hostile agent (like, a group of gnolls or wolf). All of a sudden you're managing several small skirmishes on top of the battle you were already running from. Hilarious!
I remember battling the Ogre Berserkers on the bridge in the Gibberings area. I managed to beat them, but with much bloodletting in my own party. Before I did anything to recover (and I didn't have anymore healing anyhow), a small wrong move of a character triggered that guy with his 6 hidden hobgoblins. In spite of my sorry state, I managed to kill all the hobgoblins and bring the leader to near death. The leader chased my Shadow Dancer (with 6HP) across the bridge, hoping to be able to hide in plain sight. Unfortunately, he walked just as fast as she, and in one strike put and end to her struggles. I remember this fight, because it evolved in an unpredicted manner. The Ogre fight had been quite hard, and I hadn't even prepared, as I wasn't aquatinted with the area. Losing the win like this was quite dramatic storytelling.
Anyhow! Long story short: I think it would be cool with a mod that with a small probability introduces extra monsters during battles. Like, 2-4% chance for each round of battle. The monster(s) will spawn just out of sight and walk towards nearest party member. If the battle is in close quarters, the monster could come from the nearest door.
This mod will most likely have to be combined with an XP reduction component though.