Tanks and helicopters should be able to unlock more features, like reactive armor on tanks and night vision IR vision etc etc as you progress.

For example the MG on the tank could unlock a rocket launcher once you reach a certain level.
Reactive armor that can stand more hits, smoke screens etc etc could be unlockables

As for helicopters I can't think of a whole bunch but I'm sure modern attack and transport helicopters have a ton of stuff you can make into unlocks as you progress.

Also make the G36C a weapon.
The engineers have really s*** weapons and should have a better selection of guns.

Also the RPG and the Halon shouldn't be the only bazookas.
You should also be able to unlock the Javelin (locks on tanks)

And at the end when you reach full level with the engineer you could have a next gen thing that locks on both tanks and helicopters.
I'm sure such a thing exist. This will also encourage people to play more as engineers.

Loose the Mac-10. It's worthless.

Give us more grenade options.
Smoke grenades, flashbangs, stingers that slow your movement down for a few secs, concussion grenades that blurs your vision and slows movement for about 5+ seconds etc etc

You could go Black Ops 7 as well and have modifying features for grenades, but hey I'm just tossing out ideas here.
Just a thought.

And what about giving infantry the ability to unlock night vision and IR vision (the latter to see through smoke screens for example)?
Just an idea. Not every class need to have one.