A realistic mod... for a non realistic scenario? Very interesting. While I think that what you're doing is taking a fast paced game into a slow and much more mundane game, my list of complaints would start in your movement system. I'm a normal person in relative shape, and I can sprint for 60 seconds tops. That is realistic. Of course a good idea there is to make it so you have a bar that has full energy for full speed, and at half energy the speed starts to decrease, and at 0 and in recharging, your going slower if you used the full meter. I code a bit here and there and it is not a terribly complicated system.
Now for Gameplay factors, the zombie bosses made it fair in versus multiplayer. I'm going to assume there will be no versus multiplayer if the zombie bosses are taken out. Big fun factor decrease right there.
I hate to say it but... you're reducing the fun and ways to play(if I'm correct on assuming removal of versus.) and your pretty much making it Resident evil all over again, in first person(I did not note Umbrella Chronicles because that was a rail-shooter, this is not on a track.). Not a good move for this type of game, as it is a fast paced zombie survival.
For other comments, MightyMonk and VideoPray have excellent points that relate to mine. Iron sights would probably be OK, but only on a couple of weapons. And the running zombies made it difficult, like VideoPray stated. If you make the zombies slower, you'll have to make the game MUCH harder.
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