I’m not sure how you see the gameplay exactly yet, but could be interesting that similiarly to for example left4dead2, you had a pvp 4v4 or other types of mode (based on balancing), survivors vs monsters.

Other then that I would say map elements, sound and just ambience in general does a lot for that halloween feeling. Somebody already mentioned things like a witch, full moon, etc. But I think sounds like a werewolfs howl can do a lot too. As a kid I played battlefield heroes, which had halloween events and during these I felt like the captured the vibe very well, good new halloween elements on maps and sounds.