A few ideas

I like interesting mods, but I have some ideas I lack the skills to implement. I think others might. Let me start with an outline so you can see where I am going with this.

The question I put to mods, real and imagined, is this: what changes would expand and innovate gameplay without cluttering or breaking up the experience? It is very important for me as a player to preserve SPACE in my games, which includes breathing space. Half of the beauty of the original Baldur's Gate in particular is in the emptiness of its maps so we can walk, look around and ruminate on the last encounter. Some places, like the Beregost Temple map, could receive a little more special content, subtly thought-out, but on the whole, to my mind, new quest givers should be put inside houses of places like Beregost, Nashkel or Baldur's Gate itself, most of whose houses are still "for rent," and quests and encounters themselves be tucked into scripts to happen when triggered rather than simply crowd the maps. It is better to give players a reason or several to return to a map they have already explored. (What I say here applies primarily to the first and second Baldur's Gate games, because the Icewind Dale series has a Diablo-like approach to places and Torment is wrapped tight in its plot.)

Probably the best example of this re-use principle already in a game is the first BG's High Hedge. Players visit there several times: for the quest to take Perdue's short sword from gnolls; to restore Melicamp; to do shop with Thalantyr; perhaps to kill his golems once they have magical weapons for that. If the party was full and they did not take Kivan the first time around, they might return for him later. This soon makes High Hedge a familiar and beloved destination with a lot of content without crowding it in the slightest - because the content is smartly tucked away into quests or inventories (undead skull, Perdue's sword). I think this is a principle to imitate.

Changes should be subtle, too. Having installed certain mods, I was horrified to see merchants spring up selling swords+3 and every variation on the magic bullet (if there are fire bullets, there just has got to be electricity bullets and cold bullets). While I am a seasoned player and something of a minimalist and was able to just grind my teeth and ignore such offerings, I can only imagine what this easy availability does to the balance of the game for new players. Balance affects adjacent game aspects, too. Somehow fighting gibberlings is simply not impressive when a memory of those swords+3 still sparkles in your mind. And why go to the gnolls when they are simply - gnolls? And why be mystified by the Claw of Kazgaroth when the next overpowered assassin will have a ring+2 on him and more?

The need for balance is so easy to see, it can be distracting. People are a little too eager to agree and get carried away by a heated discussion of how much is too much. For me balance is only a part of the answer to the question of how the games could be innovated and expanded yet stay lean. There are other aspects. For example, an assumption exists that all abilities and options for characters must be universal and find constant use in the game. If thieves have the Detect Traps ability, there had better be enough traps. If wizards have spells, those spells had better be broadly useful, which in practice means they have to be combat spells. And so on. While this is practical and to some extent inevitable in a computer game, I believe gameplay could be made much more diverse with some abilities and powers that only have select, special, occasional use. In pen-and-paper AD&D we had thief skills like Climb Walls and Tracking for rangers, wizard spells like Hold Portal, Wizard Lock and Rope Trick and so on.

There are, of course, hard-coded engine limitations, restrictions of the medium itself and the isometry issue. But they are not so severe as to prevent special solutions. For example, while it is neither possible not sensible to add Climb Walls as a thief ability with points, thieves could receive it as a once-a-day special power, not automatically successful. What for? Well, a modder could place a few triggers throughout the game, detectable with Find Traps/Dispel Illusions, where that skill could be used to get the thief to previously inaccessible places such as roofs or through a "back door."

Take the Gnoll Fortress. All parties must attempt a long if not very difficult storming of it up the big stairs to get to the top where Dynaheir is being kept. This is the way the designers intended it, and for most parties it should stay so. However, if you enter the same area from the north, you come very close to the back side of those weathered, ivy-veined towers just begging to the scaled. It would not break the game balance or clutter it and it would be very exciting if a thief could ascend there, then use stealth to reach Dynaheir and talk to her, pass her an invisibility potion and get out. Better yet, if the modder wanted to put in a little more graphics work and draw a rope there, the rest of the party could be hoisted, thus sneaking up on the sentries, and beat them from the back.

Now I do not expect game areas to present very many opportunities for climbing, but Durlag's Tower, possibly the coast areas, Balduran's Island might have a few convenient spots. This climbing solution should not be very difficult for an experienced modder to implement. It really takes a special ability, a trigger box and a teleport. Neither would be other special-case solutions. Why not introduce the Hold Portal or Wizard Lock spells just in case of a tough tactical battle where keeping a door shut might be smart? It would make Dispel Magic more useful too - for everyone, if enemies had the locking spells as well and used them to shut the party in or away. Again, this is not a first-resort and must-have spell, but neither was it in paper-and-pencil AD&D. By focusing on particular uses and special solutions the computer game could become more flexible and open to role-playing without the stupid and impossible burden of making all roofs climbable, making Wizard Lock an essential battle tactic for every fight and so on.

Rangers and tracking. They are supposed to be able to do this. One way to enable them is again to put some triggers in the wilder areas of the game; when a ranger is in the party and the check is made, some FX effect, like fire, could be used to sparkle along a path and a rare beast - a winter wolf, a grizzly bear, maybe even a basilisk - spawn at the end. These "trails" in Baldur's Gate itself could lead up an inn's stairs to a room for a spawn of a giant spider, to an ooze in a cellar, to a wight in the sewers. Without a ranger - no trail, no extra creature and therefore no content clutter. The game can remain pristine, coherent and calm.

With respect for those who have focused elsewhere, I think it is solutions and extra features like this that would innovate and expand the game(s) the most - not merchants, NPC, kits or banter.