The 100 Bonus AD requirement for Kai'Sa's Q upgrade is pretty harsh considering AD costs much more.
I don't think the 3 basic ability upgrades should require the exact same amount of their respective requirement to upgrade.
100% AS is extremely cheap, and more accessible with greaves cutting 35% out of that requirement, 100 AP is doable with just 1 fully built item and runes, but then getting the 100 AD requirement is like jumping over Mount Everest, especially for a champion that seems like she's meant to have a mid-game powerspike.
Even if you go full AD items, such as Cleaver + Botrk, that's not enough for the 100 AD requirement. If you go crit, ER/IE gives you a lot of initial AD, but you pretty much have to build a zeal item before or after those items for them to be any good. If you build assassin items, your dps will suffer and you might as well be playing an assassin or a crit ADC that would have even higher burst. This means your Q upgrade pretty much isn't going to happen until lategame. You'll most likely upgrade your E without even trying, before you accomplish a Q upgrade.
AD is extremely expensive when compared to AS/AP, and getting tons of flat AD on an ADC is a lot harder than getting tons of flat AP/AS (especially with rageblade likely being a core item on Kai'Sa). **
Simply lower the AD requirement to something like 80 or even 85 so that it reflects the game's higher price on AD and higher difficulty in stacking AD. **
or even an
can reach that threshold, and regardless of runes, either one alongside
, which you should be building as an ADC anyway, or
gets you there.