One of the biggest problems for AD casters currently is their lack of decent itemization, but how do you go about fixing this?

Add in more items geared towards AD casters of course, but why stop there? Why not add in Casted Attack Damage (working title) too?

But Civ, what is casted attack damage (CAD)?
CAD combines attack damage, crit chance, and crit damage into a consistent source of damage scaling for AD scaling abilities.

How does CAD work?
CAD basically works by multiplying crit damage by your crit strike chance and then multiplying that number with attack damage. Confused? Here’s an example of what CAD would look like on IE if it was in the game.

Current IE:
80 AD
25% crit
50% crit damage

New IE:
80 AD
25% crit
50% crit damage
12.5% increased CAD (essentially giving an ability with a 1:1 CAD ratio 90 added damage without considering any of the base stats of a champion)

What problems does CAD solve?

The implementation of CAD:
- Instantly creates more build variety for AD casters with the current item list
- Creates more build variety for both AD casters and AD carries when any AD/crit oriented item is put into the game. If a new item with attack damage, CDR, and crit damage (adding CAD as a result) is added into the game then it might open up options for additional AD carry builds. 1 item will essentially become 2 different items based on if your abilities scale with AD or CAD.
- Opens up new balancing options for Riot. If you want an ability so taper off in late game you can still have AD (both total and bonus) ratios, but if you want it to perform a bit better then you can give it a CAD ratio.

What problems does CAD possibly create?

The implementation of CAD could:
- Break AD casters considering it gives champions with CAD ratios a lot of stats. If need be you could scale down base crit damage from auto attacks for champions with primarily CAD ratios similar to how you give tanks scaling base MR.
- Break AD casters if you make CAD scale too well. The conversion rate for AD/crit strike/crit damage to CAD in my example is probably too strong. I just used a 1:1 ratio so better explain how CAD works.

TL;DR – CAD makes crit and crit damage much more viable on AD casters. Adding CAD into the game increases build diversity and viability for all AD champions much more than simply adding in new items specifically made for individual archetypes like AD carry, bruiser, and AD caster.

Soooo Corki graves and Ezreal all become even more stupidly powerful while kog vayne and the rest of the adc that nobody plays competitively continue to suffer? Seems legit.

Or they could add an item with a few of the following: AD, spell vamp, armor pen, CDR. Something built from Brutalizer, Bilgewater Cutlass, or some other item.

I can't tell if the people who are arguing this actually read your post or not.
From what I read it seems like you want to add a whole new ratio all together.

And to those saying it's stupid for one champion to have it and not the other.

Are you ****ing kidding me?
Talon has only AD Ratio's
"Where's his ap ratio??? omg he's so weak cause he cant build ap, that's dum of riot to only give ap ratios to sum champions."

Im not saying I agree with your solution, I'm just saying people who try to argue it need to actually use their brains.