This is probably being talked about all over the place, but fire should play a big part of 4.0. I would like to see molotovs/easy ways of setting things alight, but not as an OP method of raiding bases, etc, maybe fire for traps or for killing zombies easily.
Fire should do minimal damage, but slowly kill a player, if the player gets set alight by another player, campfire etc then they would start to slowly die unless they found a water source or bandages.
We probably will get a flamethrower in 3.x anyways, we have 3 curated maps to be confirmed coming up and plenty of people trying, we already have a fire system as well, so there is a pretty decent chance that we will get a flame thrower.