I like the FPS idea you described. It's better than simply going around a kill monsters that are in the way. What I like the most in a FPS is a huge map (really huge), with lots of little details, like picking locks, finding a tool to bust a metal door down, find amos and new weapons (or course), or acquire them from the enemy.
I like some free time (no fight), so I can investigate the area, go inside structures, roofs, underground etc. More like, the enemy is not simply everywhere, but in groups in different locations, so I can get some peace sometimes. I don't like the AI to reveal my location to the enemy 24/7 because they all go after me no matter where I am, I hate that, it's always fight fight fight. I like it more real life. If I go someplace and they see me, they can come after me, and if I can hide good, they'll regroup and let me go, basically.
I like the scenery to vary a lot. Night time, day time, rain, snow, wind, old destroyed building, new futuristic buildings, caves etc. while still looking within the look it was designed to be, like in 1942, or year 2350, or even 5000 BC, all of which have a unique look, but still can have a lot of variety.
I don't like and never will like online FPS. Too many either super good players, or cheaters. I like to play at my own pace without the need to be connected to the Internet.
I don't like to be killed and have to restart a "level" over and over. I give up. What I like is to be captured and made prisoner instead, and it's up to me to find a way to escape, never die, the game goes on forever.
Now if you make a game like this, I'll pay \\$300 for it, because I know I can play for months, and really be in the game.
Another thing to consider is crossed eye 3D capability. This Raylectron.com render does it, and it's absolutely amazing (go to their forum in the gallery, 3D). It looks so real, better than 3D glasses. Now I would pay \\$500 for a game that can do that. Now that we have very wide monitors, it'll be easy.