Great thought experiment,
Lagole Gon. Some interesting ideas so far.
If I was put in charge, I'd make something along the lines of...
No POZ, obviously. Perhaps you could get to slaughter a few pozzed characters to even things out though.
OD&D ruleset (3 LBBs only, which means Fighting-man, Magic User and Cleric, except I'd also add the Thief class as an option).
2D elements only as Baldur's Gate; isometric fixed camera.
Gorgeous artwork, painted by hand by actual artists which means no shitty digital painters - REAL paintings and drawings only. This is then scanned and imported into the game. It doesn't hurt if the resolution is limited (no plastic-looking HD graphics allowed).
The artstyle itself should be as far removed from the garbage of the 3rd edition and onwards as possible. Realistic designs for clothing, weapons, armour and locales, preferably as much down to earth as possible.
Extra attention would be given to the death animations, which should be realistic, gruesome and varied. The aim would be to surpass Fallout (1997).
The PCs would select their portraits from one large gallery. NPC portraits come from another. Only characters of import has a portrait. Plenty of voices to choose from.
The music would perhaps be a more ambient "dungeon synth" or similar offering, with tunes fitting with the enviroment.
The story would be about a party of adventurers who are in Icewind Dale to get rich by plundering the countryside before the short summer is over. Any connection to earlier games would be minimal. The overall atmosphere would be grim and unforgiving and a struggle against the terrible cold would play a major part. The entire campaign would be just that, a campaign, and exploration would take place on a nicely drawn world map. If you don't have enough supplies you will freeze to death, there would be a good possibility of getting lost, plenty of random encounters and not only with monsters, etc. The adventure ends when the player is either thwarted, gets on a ship to leave Icewind Dale or as winter sets in. As long as you have enough coin to leave by ship, you could as said do so at any time (when in port) and a tally and a high score list will appear, detailing your exploits. Of course there would be story details as well - how the PCs actions changed events in the region. If the player reaches certain thresholds of wealth, different aftermath screens an short films would play. The lowest tiers would simply have the PCs die in poverty or freeze.
The party would be up to eight 1st level characters (though as low as four would still be viable) who would all be created by the player. Attributes and starting gold would be rolled once from top to bottom for each character, and then you pick a class which suits him. As
Lagole Gon wisely said in the beginning of this thread "the main randomization is done at the start of the game, preventing savescumming". That will include attributes. Level progression for the entire the adventure could perhaps go as high as 6 or 8 as D&D is at its best at low levels (and anything above 2HD is heroic anyway, as per the original OD&D / Chainmail vision).
As PCs will suffer from attrition, there would be some opportunity of getting new 1st level PC reinforcements throughout the campaign in a limited fashion (chances dwindling as winter nears).
I would make it so that avoiding or fleeing from unfavourable encounters will be a viable tactic, as it would be in real life. The morale of enemies would also play a significant part as it did in the old days - encounters will be over when one side flees or surrenders and last man standing fights would be few and far between.
While there would be no joinable NPCs (like Imoen etc), you could hire NPC warriors during your adventures. You would only be able to give them general commands (defend, attack follow etc) and if treated poorly they might desert or even attack you if they think they can win.
There would be no "loot drops" from enemies - if someone has a magical spear, rest assured that he will use it against you until defeated. As for enemy types it would be whatever fits the enviroment - humans, goblinkind, wildlife, and so on. A game like this isn't complete without haunted ruins, covens of witches, giants and dragons, obviously, so there would be a few dungeons too, most of them not too deep, but one or two large, sprawling ones would be interesting. All would be hand made and as atmospheric as possible. Some would be impossible to find without some luck so that the game remains replayable.