Since the hardener nerf, fighting tanks no longer feel like an overwhelming challenge. I can personally damage a tank enough with advanced swarms to make it retreat in fear, and with one or 2 more other AV players on the field we can manage to completely destroy a tank. In my opinion, this is how it should be. Sometimes I can even solo a militia tank if the driver is dumb enough to stick around. Tanks are much more expensive than AV, so I think its fair that solo-ing a tank should not be a common occurrence. I also don't think it that the price tag justifies taking an overwhelming force of AV to stop one tank like some tankers would like; while AV is much cheaper, it also requires teamwork, means less people on your team fulfilling objectives, and (with the exception of the commando) also it requires you to sacrifice the use of an anti-infantry light weapon. Anyway, I think where we are now is pretty close to the ideal.

There is still things that need to be fixed with vehicles:

Repair-tanking leads to near invincibility. I found this thread on the features and ideas board that shows how crazy it is. It basically means infantry don't ever have a realistic chance at destroying certain tanks and dropships. In one of the comments in the thread I link, one pilot shares a story of how it took clever coordination between 4 assault dropship pilots (including himself) to take down just one enemy assault dropship because of how obscenely effective repair tanking is.

Large blasters are still way too good at killing infantry. A giant plasma high ROF machinegun mounted on a tank to easily mow down infantry and rack up a ton of kills seems inherently overpowered, there is a thread on the subject here. I would prefer if large blaster turrets had 30% less ROF, but 30% more damage (hell I would fine with 35-40% more damage); the DPS would remain the same, but it wouldn't be so overly powerful against infantry. Do blasters really need as much range as they currently have? They can hit you further than swarm launchers can lock-on.

While this does not affect AV, at least not directly, railgun tanks should not out-DPS blaster tanks at close range. Their should be a tradeoff between range and DPS. You wouldn't give a sniper rifle more damage per second than an assault rifle, likewise you shouldn't give a large railgun turret higher DPS than a comparatively close range large blaster turret. At close range the large blasters should generally win, and at longer ranges the large railgun should have the advantage.

These are my thoughts on the issue, and balance seems fairly close.

Its not anyway balanced. dedicated AV should always have the advantage over tanks in a 1v1 context, its what the role does.
Pro AV forgegun fit is expensive. over 250k isk each, its super weak vs all other infantry and it is the slowest moving largest hitbox most easily detected suit in the game that is vulnerable to all weapon systems. A proto av forgegun should have no trouble taking out a proto tank 1v1 if the forgegun gets first round hit and has the high ground. Proto swarms should take a maximum of 2v1 tank to have a almost guaranteed kill vs enemy tanks. Any more then that and the entire games match balance becomes unworkable.

Now either this means you reduce tank and av prices so tanks get used to dieing more and get rid of this KD invulnerable attitude. or you raise the price of the AV and tank fits and Largely raise the rewards for killing tanks so its work the extra effort. or you create a better system for tanks to be in the game, with cooldowns for calling in tanks during the match, and other features.

I want to put this bugbear to rest, because it's one of the stupidest fallacies that continues to rear it's ugly head in these discussions.

CCP has stated multiple times that they do not balance around isk.

At most potential effectiveness for isk scales logarithmically with perfect tactical choices and perfect strategic application.

To explain, since some of you are unfamiliar with the reasoning. Balancing around isk or any other number leads to an arms race that quickly reaches a static balance of an ultimate min-maxed solution. For a truly dynamic meta, you need a rock-paper-scissors type balance where increased strength, effectiveness, flexibility or other advantage comes as the result of an ultimate and non-negotiable weakness or limitation.

For a positive contribution to the discussion; I would support more options, other than raw damage, for infantry AV. Deployable area denial along the lines of prox mines. Say AOE stasis webifiers to stop a retreat, or energy neutralizers to deactivate hardeners/reps; Movable tank barricades to deny access and limit mobility; and those deployable personal shields CCP promised back when cloaks were first talked about. How about expanding hacking to not just taking nullcannons, but to also raising and lowering barricades?

And indeed we do.

You mentioned that small turrets should be purposed for AI instead. I don't really see how that will work, considering that it's the turret type that determines what it's good for, not its size. Its size only determines which vehicles can use that kind of turret and to what degree of effectiveness.

The turret types are as follows:
Blaster
Railgun
Missile
Autocannon
Artillery
Beam Laser
Pulse Laser

The turrets with high RoF are italicized. These are the turrets that will have an easier time hitting infantry than the other turrets. There's nothing that you can do about it. I'd be very interested to know how you can make their different sizes have different roles, because to me, it's the turret type that determines its role, whereas turret size determines fitting ability and effectiveness.