Ah!~ but keep in mind that levels in Dx were, well, i guess 'multi-tiered' is as good a description as any. Example: one of the early missions, in hell's kitchen. The actual area contained in the mission was no more than a couple of city blocks, but there was a hotel, a clinic, several enterable buildings some with several stories, the streets, the sewer tunnels, part of a subway, storage rooms, an underground bar.... there was a LOT packed into a relatively small space.
It wouldn't be the same if you restricted the player to the equivalent area in ofp. Since there is not really any way you can expect that AI to cope on the second story of a building, for example, most if not all npc interactions would have to take place on the ground level, otherwise you'll have loons getting stuck inside walls, running off rooftops, et cetera. This would result in 'cramming' of npc s within a small space. In addition, the player movement speed in ofp is faster than dx. What do I mean by this? Well, the time it takes you to run the length of a building is about half a second in ofp. There is nothing meaningful you can have or do that involves a long time being spent inside a building, because of this. And, of course, even the most detailed large buildings in ofp are really not that detailed inside.
So, you'll really need to spread the mission areas out a lot more. I'm not saying do away with city areas and buildings altogether, but you will probably need to keep the action outdoors mostly. Obviously.
Also- re augmentations. If it were possible, I don't think augmentations would be a bad thing to include at all. However as you say they would be quite difficult to script