The Ronin isn't as flat as the Universe, but offers a fair amount of protection at light enough of a weight. The Tarther is good and the Adios is good. If you didn't like the heel height on the T6, then you probably won't like the Adios, either. The Ronin and Tarther are both flatter, though they still have worthwhile cushion. The Ronin provides a touch more support than the Tarther. Both the Ronin and Tarther are fairly accomodating, so if you find Nikes narrow, then you might enjoy the freedom of the Ronin and Tarther.
If you like flatter shoes and don't think you'll blow out your calves or Achilles, then the Hyper Speed is pretty great, as it is pretty flat, but with ample cushion.
The Universe is the lightest and flattest shoe you'll find. I can't imagine running a marathon in it unless you've gone barefoot since birth or something similar.