Should there be more mpen items?
Armor penetration has 3 flat pen from dirk routes, 2 %pen last whisper routes, and the black cleaver shred. Magic penetration has 2 flat pen boots and Guise, 1 %pen in the Void staff, and Wits End gives shred.
Wits end isn't really a proper apc item but cleaver was never meant for adc so that's probably fine. The flat mpen route gives health, a defensive stat, while the lethality (formerly armor pen) route gives nothing but murder The %mpen route gives AP and pen, the %apen gives ad, pen, and a passive to aid in murder
This particular tree feels like it should be mirrored between ad and ap more closely. Is it just me?
. Lets run down them pen boots for 1100, Liandry which not only give armor pen but does current percent health dmg perfect for tanks gets doubled when slowed. Then their is void staff which isnt BONUS mr but just straight pen. Lets also take into account that I think most champions, but yas, have a .5 mr scaling per lvl. Thats like 9 mr at lvl 18?
( which wont even have mr next patch) 
I think i missed some you can add them. So most adc's dont even buy eon, or maw which i dont know why so lets take Draven and look at his build and how much mr he has alont with scimitar cuz that almost the only mr item he builds, 12 from runes 8.5 from scaling and 35 from scimitar and 30 base = 85.5 now show the mrpen items 35% removes 29.9 so 30 or that 65.5 masteries and runes remove 13.83 so 51.67 - 30 from boots and liandrys = 21.67 mr left with all pen items. its more then enough to get through squishes, ppl dont even build all three and still destory squishies, because in most cases they dont even build mr items especially in lower ranks.
thats 141 armor at lvl 18 compared to his mr build I even forgot armor runes which are only 9 armor but forget that its not even much.