There could be a new mutation category: smart zombie. It revolves around blending in with zombie, gives you all of the freedom you want in exploring infested area, but humans might see you as another z to kill. Serum will be found in labs. Mutations:
Slow metabolism->Very slow metabolism-> extremely slow meta: greatly reduces drink, food need
Zombified: zs ignore you (z pheromones for free, last forever), humans might shoot you. Wear heavy headwear might hide your face successfully
Utterance: greatly reduces your speaking and communication ability
Eater of the (Un)Dead: can eat rotten and tainted meat like it was normal. Eating normal meat=fresh, fresh= big bonus to morale
Think of more?