I think the best way to improve the melee and projectile system would be to change how the reticule functions. I would like to see an expanding and contracting tracking oval that dictates where your character is going to hit and tracks parts of zeds like the head. The reason I want to see this is because it helps simulate how your character reacts to the environment. It lets game design move away from damage and weapons stats and instead lets you focus on building a character's style of play.
Let me give you a scenario. You're a fresh spawn and need a wood club. It's going to break quick, but it'll get you a couple kills. With the reticule system, you're going to see that it doesn't expand very quick and contracts faster than an iron club. You could be out of breath and still accurately hit in the head with a (tiny) bit of patience. Yeah it isn't hitting hard, but normal zeds will go down with no issue. Let's say you're cold and kinda stiff. You're not going to be able to swing as accurately with frozen fingers. Or you're hot and sweaty and don't have a great grip on the club. There's tons of factors that could make it more interesting to swing the club at the head, and this makes the weapon skills better than just for damage.
Another scenario. Say we found a pistol and some ammo. Right now if you aim at the head, it's easy to miss because of the head bobbing that zombies have. If we had a reticule that bobbed with it as a sort of visible compensation based on skill, wellness (not that stat, but just general character wellness) and the kind of buffs you have (like coffee will be better for a gunslinger than alcohol unless you build your character that way), it could be a far more immersive system where our character has an effect on accuracy. It can also help reduce the frustration on wasting ammo because of head bob.
Again, back to the pistol, we get trapped in a corner by a sleeper we didn't see and are stunned. Our aim is going to be all over the place, but because out character knows to aim for the head, we can get a couple of shots off just because our reticule automatically tracks the head. Stuff like pistols and SMGs will get the most out of this at the early levels.
The best thing about this is we can override this as players by simply aiming in with weapons we can aim. Plenty of players can already aim better than you'd get at early levels so the tradeoff works out nicely.