AD Champions have four great keystones they can use depending on what exactly they need

AP champions have one, unless they're named Swain or Brand. Hell, the champion who Deathfire Touch is best on is a goddamn ADC.

The point of the keystones was to give the player a choice of which exact thing they wanted. AD champions get that: they can go TLD if they want burst, Fervor if they're planning on being more DPS focused, Warlord's for sustain, and DFT if they're Jhin. AP champions basically have to pick Thunderlord's regardless of whether or not they're burst oriented, because they only have one other option and that option is intentionally weak in order to not be overbearing on champions with DOT abilities, at the expense of everyone else.

All four offensive keystones interact with AD in some way. Two interact with AP, and DFT is bad on almost every champion. If the two ability focused keystones are going to have AD ratios, maybe the two keystones that are currently AD exclusive should be modified to work on mages. Rework Vlad and then give Warlord's spellvamp. Maybe even give Fervor an alternate mode that does stacking magic damage on ability hits if you have more AP than bonus AD.