Riot has actually made several comments about this on several different threads; I don't have those linked, so I'll share what I remember if you do me a solid and don't take my word as 100% correct :)
But, what Riot has said was already touched on by "It Hertz When IP." DFT was bad, and they don't want to revisit that. They have considered adding some AP options to the "Precision" tree, but it's a little sticky how exactly those would pan out. There weren't many details given.
Back to my take here: It's difficult, particularly because DoT doesn't have a lot of interactivity. If Swain hits you with an E, DFT is super non-interactive. Keystones aren't just about power, it's also about fun factor, and DFT wasn't fun. We're not getting that back.
If you want a keystone that makes DoT fun, the best way to make that happen is to suggest ideas that sound fun. The reason there isn't one is Riot hasn't come up with anything that sounds healthy and fun. If you have suggestions, I'll all ears, and I think Riot is, too. Like I said, the "press the attack" proc has been considered, but again it's pretty non-interactive for many of these champs (like Singed or Swain). It's hardly a great option if we're fun-searching and not just power-searching.