I have to say the majority of my time spent playing Halo 3, and Halo Reach was playing infection in customs. It was amazing but could be alot better with some additions. In Reach Bungie removed alot of good features 
so heres a chance to redeem it’s pride 
First off, effects.
-(forge) add a weather effect that starts off bright, and slowly gets dark for some good zombie feelings
-(forge) add more light breakable objects, like pallets. This would be for humans to block off intended entrances for a better survival chance.
-(forge again) add some scenery like blood, or blown up vehicles to really set the mood. I have used many tricks to make things like already blown up vehicles in my previous maps but an already set one would be nice.
-(all of these are forge ok?) In Halo 3 equipment was really cool because you could throw things like trip mines. Their absense in Reach was dissapointing and I would love something like that to come back for custom games sake, maybe even more elaborate equipment.
Physical Traits.
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I think things like jump hight/speed were nice, but alpha zombie was really cool bring it back!
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Add the trait to not pick up certian weapon types, for example: zombies have rocket launchers (idk why just go with it) but you want humans to pick up weapons placed for their use, just not the zombie’s weapons or vice versa. So you disable “heavy weapon pickup” for humans. See would work great
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sense armor abilities and “perks” will be in the game, you need to have ways to lengthen their cooldown or turn them off ext… Maybe even go a bit deeper and have a feature to only use YOUR armor ability a certain ammount of times before it’s not available anymore (unless you pick up a new one, same rules apply for already placed abilities).
Interactive objects
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Many tricks were used to create toggable doors, or button doors in the previous games, but they were difficult to produce. So the addition of some actual buttons or something along those lines would be really nice.
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vehicles like the elephant were really amazing (especialy when people got it to fly omg that was hilarious) Please bring back a few map specific vehicles like that, that can carry more than 3-4 people. This was great for infection.
Thats all I can think of for now, I am sure theres lots of other things that people want but this is my wishlist. Please feel free to add some ideas if you like.