Well, if you want to make regular shamblers a threat you have to introduce environmental threats. Like a twisted ankel due to jumping off a building slows you down and suddenly makes you feel like the slowest anemal in a herd on a nature documentary. Also barbed wire, electricity, other peoples makeshift defences. Even those spinning blades made from engines like in half-life 2.

Also, if you want to make it interesting, special infected. Not like L4D, but maybe like runners from 28 Days later or half life 2, combined with a few that are still wearing riot armour or bullet proof vests.

Combine that with the ever present danger of infection and an extreme lack of advanced weaponry. Also lack of vehicles due to highways being clogged with empty cars. Also objectives should take them into the city more than just scavenging, like a vaccine or a member thats trapped. I quite like Resident Evils take on this with the Umbrella mercs being another threat...

Just some thoughts that come to mind.