Here is a list of things I think should be added to the game. Not all my ideas, but culled from several threads on this board:
1) Kits acquirable in-game through trainers, mentors and/or magical books. Finding out where these trainers are and/or acquiring access to them could help drain the party of excess cash. If you don't have the money or don't want that particular kit, you don't have to go after it or spend the money.
2) Languages. Your main character should be able to learn certain monster languages at the start. Ones like Orc, Gnoll, Dragon, etc. This would make dialogue in those languages show up on the scrolling screen at the bottom, translated into English/Common/Whatever language the game is being played in. This way, you don't have to add more voice files to the game. This comes from the character being raised in Candlekeep, and presumably having access to all those books in their library. It also gives some kind of nice thing for having a high INT that isn't about combat advantage only.
3) Add more Dragons to the game. Not just big, nasty adversaries to fight, but smaller Dragons in places that could conceivably house them (like Green Dragons in the Cloakwood Forest, Deep and Shadow Dragons in the Underdark) or Dragons that could serve as mentors, quest-givers and kit teachers. depending on how they are approached. Add Good Dragons, like Copper, Bronze, and/or Steel Dragons (the last being exclusive to the FR). Quests to/for those Dragons would be another plus. Young Adult Green Dragon in the Cloakwood #1 tells you to kill Young Adult Green Dragon in the Cloakwood #2 for him/her and it will reward you. If you do, though, being evil, the first Green Dragon attempts to kill you, now that its sovereignty over the Cloakwood is assured. Needless to say, the party will ensure it does not win!
4) Non-magical bundles, like gem bags, missile holders and potion belts, should be added to BG1, but suitably expensive. Perhaps have one or two drop after a suitable fight (like the Cloakwood Mine).
5) Deity specific spells for Clerics of that deity, castable only by Clerics of that Deity. As it is now, Clerics, no matter who they worship, unless they happen to be Druids, are pretty much interchangeable. This shouldn't be that way.
6) No ending of a Romance by choosing to become a God. You should be able to select a companion/lover to take with you and make immortal at the end of the game.