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It's true that Fraying Sanity spawned a new archetype, possibly even a good one. However, I got plenty wrong with the first draft.
First off, Bontu's Last Reckoning simply isn't a good card. I had envisioned getting a three-for-one, triggering Fraying Sanity for three, and then my opponent not having enough follow-up pressure to outrace the mill. In practice it's too slow.
The maindeck creatures aren't much different from the sideboard creatures. What I mean is Manic Scribe, Minister of Inquiries, Fathom Feeder, and Thing in the Ice are all cheap creatures. In sideboarding, I want to be able to strand my opponent's situational removal, not guarantee them more targets. The point of a mill deck is to attack on a different axis.
The most powerful cards in the deck are Startled Awake, Fraying Sanity, and Ipnu Rivulet. The rest of the deck should help you find those cards, live long enough to assemble the combo, or be more mill cards to nickel-and-dime with smaller mills.
Since starting off with U/B Mill, I considered I searched for other playable cards in blue. Then I looked at other color combinations and how they'd complement the blue base.
It's really hard to justify creatures when you have such an extremely small chance to ever win with combat damage. They're good to block with, depending on opposing creature suite and removal configuration.
Champion of Wits looks a lot better on the back half than the front. If you have a lot of cards in your deck that function better from the graveyard, like Geistblast, then maybe. As far as possible late-game power goes, the mill deck has Persistent Nightmare and Deserts to sacrifice.
Torrential Gearhulk could be part of a transformational sideboard, but I haven't gotten than far in my testing. You want a bunch of lands, so it probably pairs well with Engulf the Shore. You can't target Startled Awake, since it's a sorcery, so you'd have to play some number of Hieroglyphic Illumination and/or Glimmer of Genius to get your money's worth.
Nimble Obstructionist would fight planeswalkers more than anything. Most people will instantly ultimate their Liliana, the Last Hope or whatever, which is great to stifle. Otherwise you can cast it as a 3/1 to attack planeswalkers with lower loyalty.
Fraying Sanity is so important to winning the game that it's worth it to spend early turns to find one.
Pieces of the Puzzle can't find Fraying Sanity and, depending on the build, you might not have enough instants and sorceries to justify playing it. Anticipate is weakened by the fact that you don't have very much else going on at instant speed.
Counterspells in general are in a tough spot in the Mill deck. Most of your deck operates at sorcery speed. You get into tough spots where you have to decide between leaving up a counterspell or playing Fraying Sanity, or waiting until you have the mana to do both.
Censor deserves a place in most builds of Mill. Cycling is strong as simply more looks for your strong cards. Supreme Will is kinda pricey, but should probably end up in most builds too depending on how many of your other cards can find Fraying Sanity. You'll be using the Impulse mode more often than not.
I like Dispel and Invasive Surgery, depending on when I'm concerned with discard and counters, respectively. A turn 4 resolved Fraying Sanity is usually acceptable, while a turn 5 Fraying Sanity is much harder.