even tho some champions dont rely much on auto attacks as their main source of damage, for ME that doesnt make them ad-casters.
riven definitely isnt an ad caster and zed and talon arent aswell. they are clearly assassin-types and even tho their damage comes from abilities, the use of said abilities are clearly not spell-cast-resembling for me.
even tho talon could barely pass due to him at least having 2 somewhat caster-ish abilities, but his main damage still comes from auto attacking and auto attack resets and all of his spells are basically instant, so there is no concept of "casting".
for jayce and even varus i would say they both can pass as ad-casters but since they both have extremely good waveclear and long range, they have many options to build and to choose their powerspikes freely. many itemizations possible, so there's also no real need to do anything about it. they are fine.
really the only one who comes to mind as a true ad-caster in need of help is pantheon as he has almost no waveclear, very low range and he has to stand in close vicinity to his enemies and actually CHANNEL a spell to deal his main damage (wow, that is ad-caster style at its finest), this makes his itemization very limited.
black cleaver is a decent item on him in theory but he actually doesnt want the health and the %shred is mediocre too. now what else is there? he needs alot of high, raw AD and there is no item in the entire game that consistently gives cheap high AD without any other added bullshit. the ONLY "big" item in the ENTIRE GAME that has no downside for pantheon is last whisper because it gives him armor penetration and AD, exactly what he needs.
all other items pantheon likes alot all have downsides. as cool as hydra, essence reaver and bloodthirster are on him, those items live on their on-hit effects and pantheon at his best doesnt auto attack much. very little at best, so all these amazing on-hit effects go to waste.
infinity edge also does nothing for him as his main damage (E) doesnt crit, he doesnt auto attack much and his Q only crits when hitting enemies below 15% health and it has 100% chance to do so, so even the crit chance on IE is lost. maw and scimitar have added defensive effects that, even tho very good in some situations, are still expensive in their own right.
pantheon just has a very hard time buying raw damage without spending money on, well not exactly useless but definitely less usefull utility. the brutalizer is amazing for him but sadly there is no real "upgraded version" of it. now with the new jungle items, warrior is an actually upgraded brutalizer and made pantheon stronger again. tho this is the only item like that and even that is not very strong and slot efficient.
so, what would a very cost-efficient pantheon look like? warriors, last whisper, ionian boots, BFsword, BFsword, BFsword....
there simply are no usefull ad-caster items for him.