Have played the format a bit, thoughts on stuff in this thread:
-Pain Seer is ok with some backup.
-2/2 or 3/1 is probably fine, 4/0 wasn't common in the successful block decks but I don't think it can be too wrong.
-The one drops all suck at attacking into Caryatid/Courser, their saving grace is all the removal sucks against 1-2 drops. They are best at getting sac'd to Butcher of the Hordes. Mono-black and mono-red are still fine on some level.
-Thrill-Kill Assassin will be missed somewhat. The 1-drop deathtouch morph is good. Always trades for at least a card, can trade off for aggro creatures or just get in chip damage against the green mana guys. There's a lot of cheap deathtouch guys if that's the effect you want, actually. Also Bow of Nylea.
-x/3 for 2, x/4 for 3 is pretty common. 5 toughness is valuable because of Stoke the Flames, 4 toughness guys hurt Bile Blight and Lightning Strike (existence of Mantis Rider and Xenagos means Lightning Strike still has value in more midrangey matchups)
-can usually do something better with your mana now, Herald of Torment is probably the most worth running? Leafcrown Dryad is close since it's a 2 that eats all the aggro dudes and bestows for cheap but it's probably not good enough.
-Master of the Feast is in a weird spot, 5 toughness is great, being an enchantment is bad. Personally don't think it's very good but it ends games quickly.
-Grim Haruspex is sweet. Plenty of good synergies to play with as well as just killing their guys and trading guys off in combat.
-Flying (Mantis Rider, Butcher of the Horde, Sarkhan... Vaporkin...), big creatures (Knuckleblade, Anafenza, Polukranos), go wide (Sidisi, Mardu Ascendancy), removal (Stoke the Flames, Hero's Downfall, Devouring Light), go big (Elspeth), go bigger (Hornet Queen), go bigger-er (Ashen Rider), go biggest?? (Stormtide Leviathan????)
-blockers? Rabblemaster + Stoke are like the only actual cards in the small monored decks so uh kill Rabblemaster on sight if possible.
As for the whole midrange thing, so far the midrange-y decks our locals have built have been run over by the extreme-manabase 3 color big-aggro decks but I'm pretty sure they are just building them wrong. That said, there are games where, say, the Temur guy goes elf -> courser -> polukranos and just dies because casting 1 spell a turn isn't enough.
Mardu 1-drop 2-drop Rabblemaster Butcher is one of our more successful decks at this stage, Jeskai Ascendancy aggro/"tempo"(burn) is the other. Reanimator also putting up good results. The Mardu deck is probably the closest to "good" since it's pretty paint-by-numbers in terms of construction.
Stock Naya and Temur were pretty underwhelming, all the Abzan midrange decks suck. I hate the lists people have been bringing for a lot of reasons (not enough removal, can never hope to cast more than .75 spells a turn, endgame isn't even better than the aggro decks, etc.)
Courser is likely worse than it was in Block ; still way too powerful with fetches to not be a key player right off the bat. The better your deck is at not caring about Courser, the better.
Dig Through Time is sweet. I think Treasure Cruise is slightly better due to U vs. UU being more relevant than sorcery vs. instant a lot of the time.
-edit-
elaborating on the bestow point, Herald is the only playable one that gets an x/2 to 5+ toughness to not die to Stoke after the fact. The tempo loss of going for a bestow and getting blown out in response may not be as bad as also getting 2-for-1'd but you can't really afford to get Time Walk'd very often.
I guess Nighthowler is also playable in a self-mill deck which is probably serviceable.