So, Tank Ekko has damage, utility, and tankiness. While not totally inaccurate, there is a good reason to build AP already, that being damage and heal/shield (or tankiness). In fact, here's a rundown of his AP ratios:
Passive (80% damage per 3 hits)
Q out (10% damage)
Q in (60% damage)
W passive (3% damage in %HP when below 40% HP)
W shield (150% shield)
E damage (40% damage)
R damage (130% damage)
R heal (6.6% heal, in %HP)
And with all of that, you want to pad his AP scaling more? That's 150% AP in shield, and 6.6% AP in %HP heal. That in itself is pretty darn tanky, if done right (obviously). And damage Tank Ekko has damage, but clearly AP Ekko has the advantage on that. Utility is just there, and not necessarily hard to land with both builds having some forms of tankiness. I wouldn't say either build is truly lacking in any of the 3 areas.
Ekko's tank build does make applying his kit incredibly simple and risk-free, basically never having to worry about his tankiness between his build and his extreme utility, and still applying fair damage, particularly if you can power down HPs below 40%. However, AP Ekko gets remnances of that tankiness while absolutely killing it with damage.
So, where does this AP ratio proposal go wrong? Well, right now, Tank Ekko trades off damage for tankiness, and gets reliable utility out of it, whereas AP Ekko makes essentially the exact opposite trade (less tankiness and utility, more damage). If his utility were sapped in the tank build, his only advantage would be the tankiness, as the reliability of applying his utility would be scratched by the power of utility in AP Ekko. So, Tank Ekko would trade damage for basically nothing, and AP Ekko would really trade about nothing for high damage and higher powered utility. Right now, Ekko has 2 sharply different modes of play, and shifting utility to AP would only combine the 2 into an even more OP frenzy.
Stop trying to buff AP Ekko. He's not necessarily overly OP (questionable), but he's definitely holding his own. What needs to change is this assumption that Ekko's utility needs to be as game-breaking as it is. He has an extremely powerful assassin-type ultimate, and a second skill that basically functions like a typical AoE utility champion's ultimate (W), and both work just fine as is without any investment either way. Those two need clear paths to take, and the answer to both should not be AP.
And keep in mind, his "2 ultimates" only take into account 3 of his AP ratios, only 1 of which an actual damage ratio. There are still 5 more AP damage ratios that have nothing to do with his two quasi-ultimates. While those 2 are heavy factors in his kit, he still has plenty else to work with.
TL;DR: Don't give Ekko 9-10 AP ratios before he even gets a Lich Bane/Luden's Echo.