I would like to see situations where having a high charisma actually helps you - or where having a low charisma isn't always worth it.
I would also like it if the weapons and armor had more unique sprites - a suit of plate mail should not look different on an elf than it does on a human or an orc, for example.
I'd like to see dwarf, gnome, and halfling sprites that don't all look the same - and I'd like the half-orc to look different from the human.
I'd like some additional race options (some subraces, the option to play as a full Orc, goblin/hobgoblin, etc.), with consequences (both good and bad) based on your choice of race.
I'd also like to see the class kits balanced for play in BG1 (they were designed with the assumption that you'd be starting with them at level 8/9, which meant for a number of them that starting at level 1 gave you no abilities at all).
I'd like to see some more robust stronghold quests - a Thieves Guild quest that actually ends, for example. It would also be nice to have some stronghold quests for the different warrior classes, or variations for each one, since there are variations of the cleric quest based on alignment. A monk stronghold that's a monastery somewhere in the hills? A barbarian stronghold that's a battle arena somewhere in Athkatla? Also, the paladin quest ought to be longer - or it shouldn't prevent you from pursuing the De'Arnise Hold as well, since it's so short.
I'd like to see some inklings in BG1 that the plot of BG2 is coming. Some references to an elf who betrayed his people, or some rumors that "there's a guild war in Amn".
I'd like to see more character-initiated quests in BG1 like there were in BG2. There are a number of NPCs in BG1 that exist in your party but have no real plot arc.