lootable dead bodies in houses that could be zombies (some bodies should be zombies, some should not just for fun!). there could be ratio of bodies, for example: zombies:dead=3:7 or z:d=1:11 basically there should be less zombies than dead bodies (this way player may try his luck and loot a zombie, but if it were the other way "new" players would probalby have to avoid them, and more/less geared players would always damage lootabe items)

if they are zombies, they should be "woken up" by sound, so if you want to go pass them unnoticed you should walk while crounched, or in prone
also if you loot a "sleeping" zombie, the action of looting should wake it up

if you choose to run away then the zombie would (of course) chase you, but if it chases you down the stairs, zombies legs should get broken (rotting corpse and that....)

those bodies could have a weapon of some sort, with some ammo in it (between half of the maximum ammo and 0, but with few lucky spawns with more than a half)
for example: they went down fighting (the weapon still has some ammo), the commited suicide (there should be no ammo or should be full magazine with only 1 round missing)
(should apply for all weapons, bodies/zombies could be in civilians: random items and weapons, could be military: have military grade gear and medical equipment, all of them should have some sort of handcuffs, police: less chance of automatic rifles, but other as same as military, possibly medical staff in hospitals: 80% of them should at least have a bandage, some of them could have a bloodbag, full medkit, 50% or less or more could have a blood test, morphine... and so on, fire department: they should have some kind of an axe in 60% cases or more, some medical equipment, some tools,..
basically what they would probably in real life)

those zombies could be people who have knives beside them (suicide by slashing), but don't have to be....