I say let critical attacks make the bolt damage increase from 60 percent to 90 or 100% AD. Now hear me out. An ADC needs the following
AD
Lifesteal
Crit
ArPen
Attack Speed
Defense
Assuming a high dmg build like IE + PD + LW + boots, it leaves only 2 items. Usually a defense item is added so say a Banshee Veil/Guardian Angel. Last item would be a lifesteal item. But depending on choice, that'd mean less attack speed or less damage.
It's a tricky balance. One that they must trade something for the other and losing one would mean they'd have a weakness. Be it less crit, less single target damage, less/no lifesteal, less tank damage (from ArPen), less attacks (attack speed) or less defense.
But Runnan doesn't add anything special to the mix. Why would I want to deal AoE damage when my AoE damage isn't that great at prioritizing and neither is it a
Anyways adding Runnan as a viable path would make ADC's think even harder about what to build. Think about it. Assuming each ADC already has a dorans blade.
Do I deal great AoE damage (Doran/IE//Runnan/Statikk)in exchange for the ability to be able to kill a single target well (Doran/IE/LW/PD)?
Or do I do both damage and good tank damage and deal less overall damage (IE/BorK/LW)?
Or am I a muramana ADC (Muramana/TF - IBG/Reaver)?
Or am I a Vayne ADC (BorK/Statikk/LW)?
Or am I a HIGH AD Caster ADC (Essence Reaver + BT + BC) ?
Or am I an ADC who needs high attack speed with sustain but less single target damage (IE + BorK + Runnan)
Or am I looking to deal with high armored target? (BorK + Yoummu + LW)
Do I need extra mobility when dealing single target damage? ( IE + Youmuu + BorK)
Do I need heavy sustain while dealing respectable damage? (BT + BorK + LW)
Now this is a 3 item build (4 if you include boots and 5 if the ADC builds a defensive item which could last for 40 mins)