Ethersworn Canonist is the perfect option to bring us to one cast per turn: as an artifact it doesn't count itself, and it allows us to cast Aether Vials without them counting as our once-per-turn cast.
Since we can't take advantage of ETB effects, I had to find some other way to squeeze value out of our creatures. After looking at many options I settled on +1/+1 counters as our value engine of choice.
And since we're using +1/+1 counters we want bodies that have strong keyword abilities, as a +1/+1 counter on keywords is better than the same counter without them. With this in mind, I selected Hushbringer as the ideal ETB hatebear. Not only does she have both flying and lifelink, but she also stops death triggers as an added bonus. What's more is that, as a 2 CMC creature, it's a trivial matter to hold vial on 2 and flash her into play to block ETB/death triggers on the stack.
Because Hushbringer is our ETB hatebear of choice, modular isn't a viable keyword for us despite our use of +1/+1 counters. This is because modular is a death trigger, and so we look to other creatures to generate our +1/+1 value.
All of our other creatures are thus selected for their ability to either generate +1/+1 counters, or their synergy with those counters. The only exceptions being Mother of Runes which is our removal protection, since keeping certain cards/synergies on the board is important to us, and a couple flex slots, namely Grand Abolisher and Spike Weaver.
I could not find one competitive deck in either Modern or Legacy that doesn't want to use the stack on their opponent's turn, so I consider a one-of Grand Abolisher to be a perfect fit for the flex. It synergizes amazingly well with Ethersworn Canonist since Abolisher brings the opponent from one cast per turn down to one cast per round. With both of those and a mother on the board it's virtually impossible for most decks to remove any creatures (short of board wipes).
Spike Weaver puts counters on the board (always good in this deck), gives us a fog that we can repeatedly abuse, and provides another way to move counters around (useful in a pinch).
Because we don't want to get flooded by our two main hatebears, we run 3x of each in the mainboard with the 4th of each in the sideboard. This allows us to reliably get one or two of each per game without flooding, and against any deck that is particularly hosed by either we're able to side in the fourth for games 2 and 3. After a lot of playtesting I can say this detail is a crucial feature of the deck.
Also, since most of our creatures are 2 CMC (with only one >2 CMC in the deck), we can very reliably leave our Vial on 2 without much concern.
This is also why we favor a creature like Benevolent Hydra over an enchantment like Hardened Scales: Hydra can be flashed out with Vial, Scales can't. Hydra can be protected by Mother of Runes, Scales can't. Hydra can be used as a mana sink, Scales can't. Hydra can pass counters to other creatures to save them from Lightning Bolts and Orcish Bowmasters, Scales can't. Hydra can feed Generous Pup, Scales can't. Etc.
