A modern FPS that:
* Would make use of medkits instead of Health Regeneration
* Would let you carry many many guns.
* Would have a variety of guns,at least 30.
* Would have secret areas to look for.
* Would have some kind of collectible in the levels to look for. (like Gold in Wolf3d)
* Would have a large single player mode.
* Would have large,some time complex levels.
* Would feature grotesque and disgusting/frightening moments.
* Would give you the feelings of lonelyness,hopelessness etc.
* Would feature some puzzle/maze like levels that would make me burn my brain to find the way out.
* Would let my character move as fast and be as agile as Duke in Duke Nukem 3d,anybody on Quake 3,or B.J. at R.T.C.W.
* Would have various gadgets that I would carry at my inventory to use them accordingly.
* It would have a mostly serious storyline with cinematic cutscenes,and maybe some humor at some points.
* Would let me aim at the enemies RTCW/Goldeneye style,not with iron sights.
* For console's shake,iron sights would be replaced with good old auto aim/aim assist.
* It wouldn't have obvious invisible walls.Better see the nothingness that is behind some walls than hit my head on an invisible wall.
* Would feature lots of unlockable content to enhance replayability and keep me interested.
* At the end of each level,it would tell me how well I did.
* It would have split screen,and system link/LAN multiplayer options available.
* Its PC version wouldn't have a paranoid DRM.
* It would make use of any extra feature of all platforms.It would make use of xbox360's Kinect in some way,PS3's 3d graphics,Wii U's controller,PC's dx11 and raw horse power.
* It would have 20 or more multiplayer maps from the get go,like the good old days.
* It would have many customizable options for the multiplayer to let us choose various things.
In short words: An FPS with modern graphics and technology that its design filosofy and gameplay mechanics would be from the classic pre-2001 era.