This would be relativly simple, and would work great for d&d nites with your buddys.

I would begin with a new engine but the same basis oldschool fallout1/2 engine at least with graphics hex grid combat variables ect. (i have reasoning for not useing tactics)

heres how games would work, The "overseer" (aka dungeon master) would build a campaign from scratch, the only needed presets are random combat encounters.

The interface is key, and should be erganomic, but still have some options for advanced users.
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Example: lets build a town.

The building the basic map shouldnt be any harder than making building a skatepark in tonyhawks pro skater. just drag drop, rotate ect. My main reason for not useing tactics because it has lighting to deal with.

interactive props like doors and switches would have a dialog box to accompany them.

Wooden Door_01
[ ] locked [lockpick 25%]
[ ] can be blown with dynamite,
[ ] has key [Mr.Smith's house key]
[ ] is trapped [shock trap]


All npcs will have a character sheet that may vary depending on race/species
the basics for all npcs would deal with inventory, combat, agression, tactics. daily routine. ect.

Npcs will have a barter option as well as a shop keeper option.
shop keepers keep their inventory in a safe or other location, as well as only trade at certain times a day.

If this idea was getting major attention i would even go as far as to have the ability to set up economy, instead of npcs just having more gear ect.

I don't understand... Is this a computer program you're thinking of for helping to streamline PnP sessions?

The Fallout PnP Community has a Wikia here:
http://falloutpnp.wikia.com/wiki/Main_Page

could work.

but this way you wouldnt need pen and paper. It would be basicly fallout co-op with custom made campaigns.