Here are some ideas I've been thinking about for the game. Don't know if they've been done before and I'm too lazy to use the search functions as some forum Nazis will no doubt point out :)
1. Pet Dogs
Pets have been proposed before but I'm talking only pet dogs because the model and some of the coding for my idea already exists
Dogs will be able to :
In missions :
Do melee damage to zombies and also maybe scavenge? as in retrieve.
In Raids :
Sniff out traps/ suppress survivors by barking and melee damage
Dogs would replace survivors on missions or maybe an extra slot for them?
Also you would need to build a kennel for them and they would have food/water cost associated with them like all survivors
Different breeds would have different specializations like pitbulls would better at suppressing german shephards would have better trap detection and golden retrievers faster scav speed etc
2. Flashbangs (also known as flingable monkey poo)
Flashbangs would be an active gear that can cause instant suppression for 2-3 seconds on survivors and can maybe stun zombies for a bit? (flingable monkey poo would look so much more awesome though)
3. AP rounds but those i know have been discussed in detail so wont go into that
they require some work but i think they could be fun features