I think the game is good as it is, to be honest.
However, if I'd made it myself, the changes I would have made are as follows.
A rooster of 16 survivor characters, never all in play at the same time though, but if someone dies and you find survivors in a closet, it should be new ones.
A lot more different maps, with different goals, and preparation maps. Supply runs, defences, etc. The different maps would also have different amounts of survivors, from 2 to 8. Sometimes they'd start in different locations, 2 of them starts in the church and 2 of them in the mall, or something like that.
I'd change the weapons completely. Remove the automatic weapons, the auto shotgun and remove the scope from the rifle. Regular shotgun, hunting rifle and pistols should be enough. I'd then add melee weapons, like axes, shovels, metal bars, etc. I'd also make a nice little script to randomize your weapon model. Pick up a meelee weapon and you get a random one of the aforementioned. Find a pistol and it's a random model, with a few variations, but all the same stats.
I'd also lower the hit points of the survivors by a lot. Like a huge lot. I'd change the meelee, so it wasn't so overpowered, you should be able to keep one zombie at bay, at a time, with a ranged weapon, meelee weapons would do a lot more damage though (but still wouldn't keep several zombies stunned), but in return add a viable stealth element, for both zombies and survivors, instead of having hordes all of the time just because you're standing still, make it something to fear. Right now the hordes are no threat, just a reminder to get going. You should try to avoid them, but if one comes, you'd better be prepared to run.
I'd also add a few more special zombie types, screamers for example, that looks like regular ones but if shot starts screaming like crazy, calling in a lot more zombies to their location. I'd also change the hunters into looking like normal zombies, but with bigger claws, and make them able to climb on a lot more surfaces and hang onto overhangs, or ledges, and jump from there.
I think those changes would make for a more desperate and thinking situation. You'd be more easily scared, at the moment the game can't frighten me, lots and lots of minor annoyances and no real threats unless a tank or a witch comes along. It's just not scary. Suspense is what's needed, a stealthy game is to me always more scary than any other, just because you have to stay hidden. Thief and Penumbra has given me more frights than any other games, just because you really have to watch out, and keep down.
The few aestathical changes wouldn't be too hard to make, either, and I think they'd really add a lot to the immersion, and thus, the scary parts.
Edit: Oh, and how could I forget. I'd make a celebrity appearance special, with Gordon Freeman, either as his usual completely mute self and have everyone pissed off at him after they heal or help him and he just walks away, or have him use sign language to say "witch!" or "boomer!". And I'd have Coltrane in it too. He'd be pretty useless and good at attracting zombies, but yeah baby, he'd kick some ass!
Adding infected dogs would be pretty cool too, in some levels. As a special encounter perhaps. Instead of a tank, there's a pack of dogs.