We have recently implemented two rules that really have made the game more engaging and thought I would share.
The first rules we made because we always seem to end up with 4 players and 8 survivors. We had long wait times when each person would activate both of their survivors one after the other. We tend to try and split the party into two groups with one survivor from each player in each group. We then handle each group one at a time allowing each player to activate their survivor in that group. This means that each player is never waiting for more then 3 other survivor to activate.
We have recently added Deadeye's, Crowz, and NPC's to our games. We have added all the necromancers and abominations. We find this challenging and pretty deadly. A simple change that really seemed to make a big difference was having food and water equipment cards heal one point of damage instead of XP.
Playing with 8 survivors increases the spawn point limit to 7. (p.37)
I agree that 8 is a lot. I've found that adding more survivors decreases difficulty while increasing the length of an already long game. I've recently been enjoying the 4 survivor quests on the Zombicide website.