I mean he could simply just not do that lol. Peoples love for the characters of automata is a clear indicator he would be a great choice to direct a mainline FF
He only has a history of making games with routes. So there's no telling if he'd make a game with one route in a satisfactory manner. And his games usually aren't very long at all if you omit the tedious busywork and/or grinding that gets thrown in after Route A.
And as for his characters... I haven't played Automata past the demo but I found the writing there to be cringeworthy at best.
Drakengard 1 is only known because it has a cast of insane characters that comes off as tryhard more than anything (psychopath, incest, pedophile, cannibal, giant demon babies) and most of the depth of the story isn't in the game itself IIRC.
Drakengard 3 was a wild ride in Route A but it was tbh mostly crude junk, and the Apostles were written terribly. Everything that comes out of Desmond(?) and Octa's mouth is an unfunny innuendo or crude remark about sex.
The original NIER was okay but I could not get myself to go past Route A because the game was horribly paced and the gameplay was bad. Not to mention that the game was boring to look at.
I like Taro's insanity but he has not given me any reason to believe that he would direct a good Final Fantasy game.
It also seems like, for the most part, he does not budget well. Drakengard 1 has like 25 minutes of good-looking/great-looking CG and every aspect of the game is unsatisfactory. Drakengard 3 has a gorgeous CG opening but the game looks like ass, is short, and runs at like 10 FPS.
And with NIER they actually wasted money making two versions of the game.