So I have been thinking, it's been quite some time since marksmen received any new items that they could use.
Marksman itemization has really improved since season 1.
It used to be
+
+
+
was the only thing you'd get, ever, on any marksman.
Unless it was Corki at any rate, where a
was thrown in the mix somewhere.
Now you've got the choice of
or
, the order in which you get items can really vary depending on the marksman, and you can divert from the typical build to a more duelist/active centered build with
+ 
One idea for a marksmen item right off the bat would be a % AD bonus (like Rabaddon but for ADs).
ADC's have critical strikes and attack speed to scale their damage to amazing heights end-game, along with armor penetration, CDR, and life steal.
APC's don't have a critical strike multiplier, CDR is a pretty universal stat, and spell vamp sucks on most of them.
The only multipliers glass-cannon APC's have is Rabadon's, Void Staff, Lich Bane, and DFG. Those are their equivalents to critical strikes.
AD Casters have Ravenous Hydra and Sword of the Divine as Lich-Bane equivalents for things that don't scale with crit-heavy builds as well, and they also have considerably more %armor shred than the available %MR shred due to Black Cleaver existing.
Giving AD another multiplier that would further increase their late-game power, auto-attacker and caster alike, would be problematic for the game. It would make late-game monsters like
and
that much stronger. Which would either make them flat out stronger, or it would result in unilateral nerfs to late-game champions to make them weaker early to compensate.
Neither of which are particular good, especially seeing as marksman are so weak early they're more or less unplayable in Treelines and Dominion. You place much greater emphasis on 1 single play somewhere in mid-game for a rapid transition between sink or swim for such champions, leaving them in a condition they're useless or OP with no middle ground.
Also - making a universal must-have item runs contrary to your point of diversity. Everyone getting the same item every game is everyone getting the same item every game.
The key to diversity is choice - offering a different spin on something like The Bloodthirster in Kingblade, an alternative to PD in Shiv, an alternate path to IE+PD+LW in Ghostblade + Kingblade.
I'd generally argue marksman are doing really well with itemization choice ATM though, it's come a long way.