Since this is still on the table, development-wise... >:-)
As I am the type of player to remove every bit of black tar from every area every game, I end up uncovering (and traveling) every section of every area, and I've noticed plenty of conspicuous "dead spots" in areas that appear as if there were already ideas afoot for an encounter that simply never got added. Filling up every corner with encounters would be overmuch, but there's definitely room for expansion within the existing BG1 terrain (not knowledgeable enough about BG2 areas). These locations just seem ripe for some area-specific quest or encounter or even an NPC-related encounter.
Mind you, by new quest/ encounter, I mean another "Caldor and Krumm" or "girl who lost her kitten" or "Lord Foreshadow" or group of raucous adventurers or old hermit or Surgeon or guy-just-wandering-the-area-with-a-tale-to-tell or the like... but NOT just another monster-spawn spot. We've got plenty of those. Instead I'm suggesting to take advantage of BG's wiggle room (and awful amount of unused space) to add at least a few more encounters of BG craziness to the wilds of the Sword Coast to make them new to veteran players (and just enhance them generally). And really it would even be great for an existing monster-spawn spot to be removed to replace it with some wonderful little adventure. And it doesn't even have to have some huge quest attached... though a small one wouldn't hurt. :-P
As just one example of such a dead spot: AREA4700- that huge open meadow south of the xvart village... just nothing there in all that space until the end... where there's just a couple bears. About it. Or the northwest section of that area- north of Borda- where it's open space, trees, some rocks outcroppings, beautiful landscape, and... as Mr. Mellish diabolically discovers... nothing happens. Not the best example...
AREA4000- the mountain top past the dwarven mercs east of the actual Gullkin village- always wished for something to happen there; looks like the perfect place for a wierd ritual to be taking place (this is really the top of the priority list)
AREA4300- Just north of Nashkell, in the south there's an island you have to struggle with terrain to get to... and it's empty... and it's got a cave entrance... that's dead... So you turn around... (this would also be up there as a spot that looks primed for an encounter)
AREA5000- the chasm maze in the south of the area with the revenant/ ghast caves and ooze mage- always intrigued me with all those twists and turns but no encounter; actually all along the south has options, and also an encircled craggy area in the northeast with nothing in it AREA4200- the house northwest of the lake- seems like you should have access to it, but... nah
AREA4600- that whole southern area- not down in the Firewine Bridge ravine itself but if you follow the west edge of the area southward you end up on the south part of the area a long distance all the way... to a dead end in the south... great place for an ambush; but also large dead spots in the southern ravine itself
AREA2800- ("crossroads") the northeast corner which you can only enter near the center-north of the area, otherwise unseen behind the surrounding ridge
AREA3600- The mod Sirine's Call adds an encounter to that lighthouse which gives it more purpose than a window telling you "you see nothing inside" (or some such)... But if the devs don't want to break that rather popular mod, there's also a peninsula up in the northwest past the hobgoblins that you pursue to its end (not down by the golem cave) which is just empty
AREA3100- There's an intriguing bit of rocks in the center east (east of the ogres) which has nothing in it... looks like somewhere potent for an encounter
AREA4900- Carnival- Lots of open tree space in the east and west, but also that silent opium hut has potential
AREA3900- that southern section of the Ulcaster area that goes on and on in dead rocky terrain... though it does somehow end in an odd kobold commando outpost which is fun tactically
AREA5100- There's a great path that heads north just after the bridge and then circles west around the north end of the gnoll stronghold walls... where the path just squeezes to a halt... And a better path in the xvart south that leads back east to the river in a nice scenic view
AREA5300- Just south of Nashkell, lots of great craggy terrain crossed by rivers and a waterfall, and lots of dead area- west edge, south, center
(BEREGOST) TEMPLE- Large dead spot along the south and somewhat the north with curious rock crags in the southwest and southeast... that are just sort of there
THIEF MAZE- That whole nerve-wracking ordeal and just skeletons and traps?