I am going to go through many archetypes, and catalog what they lost, and what immediate replacements they may move to.
Also, I will mention occasionally a card that was in the list of bannings that would be on the table for unbanning once the format stabilized. I have gone for a better safe than sorry approach, with full intent to re-evaluate once things have settled.
Jund/Junk/GBx Midrange Losses
Bloodbraid Elf
Dark Confidant
Kitchen Finks
Tarmogoyf
Straight replacements probably have it going for
Huntmaster of the Fells
???
Geralf's Messenger
Plague Leech
That said, I think BBE and Confidant had done some serious warping to how jund decks are built.
There still seem to be an abundance of good cards, I think there is a lot more argument about what color you want after GB.
Red - Lightning Bolt, Terminate, and Huntmaster of the Fells
White - Path to Exile, Knight of the Reliquary, Doran, Loxodon Smiter, Lingering Souls
Blue - mana leak, creeping tar pit, serum visions
Some cards that gain ground with the banning
Huntmaster of the Fells
Geralf's Messenger
Plague Leech
Sprouting Thrinax
Thragtusk
Eternal Witness
Primal Command
Bonfire of the Damned / Rakdos Return
Broodmate Dragon / Inferno Titan / Grave Titan
Affinity Losses
Cranial Plating
One card, but it probably shakes this deck pretty hard.
I think the pool is definitely still deep enough for them to exist though.
Excellent anchors to try to rebuild from
Etched Champion
Arcbound Ravager
Steel Overseer
Master of Etherium
Tempered Steel
Good cards to fit in decks, depending on the style
Thoughtcast
Galvanic Blast
Nexuses
Signal Pest
Mox Opal/Springleaf Drum
Memnite/ornithopter
Tempo Decks
Delver of Secrets
Snapcaster Mage
Tarmogoyf
Geist of Saint Traft
Vedalken Shackles
So, yeah, they got hit hard. If there is one archetype hit hardest, its here.
That said, I am not generally a tempo player or deck builder, so there may be next steps I am not seeing yet.
Steppe Lynx / Path to Exile / Lightning Helix / Lightning Bolt / Vendilion Clique / Mana Leak / Serum Visions seems like its still a pretty solid core, you might have to search a bit to find 12 cards to replace what was lost, but I am pretty sure once I look, I'll be surprised at the options they have.
RUG delver is probably out and out dead, unless something like Quiron Dryad rises up to take tarmo's place.
While this archetype has been very hard hit, its also traditionally been pretty easy to design cards for, and they often look deceptively underpowered at first.
Ramp Decks (Tron and Valakut)
RG Tron is probably killed, although once things stabilized, if we have some control in the meta, I would say Expedition Map and/or Sylvan Scrying could come off the banned list. They are banned right now to be safe from turn 3 karns and wurmcoils warping the meta, but once the format stabilizes, those may be fine things for a deck to be able to do, as it can be answered by the control and tempo archetypes fairly easily.
UW Tron could conceivably be reworked, along with the various other control Tron decks that have lived in previous environments like UR tron, etc.
The current builds of Valakut are dead. But RG Valakut can probably look back to how it worked in standard and build something solid enough.
Birthing Pod Decks
Kitchen Finks is a hard hit, and the breaking of the kiki-jiki and melira combos are also pretty rough on the existing pod decks.
That said, Pod itself is insanely powerful, and it could likely resurrect around something like Saffi/Crypt Champion/Blood Artist.
A slower environment like this would be could probably also find a way to leverage Pod in a more midrange fashion just to eek out value, likely with comes into play abilities like Eternal Witness, Restoration Angel, and Thragtusk playing a role.
This deck has to change a lot, but I think it actually has a lot to gain from the expected environment shift.
Infect/Niv Magus
Both lose Apostle's Blessing
This is likely one of the decks that is part of Level One of the new environment.
Assuming the format begins to slow down as expected though, and control can establish itself and make combo a little less prevalent, the decks that these prey upon would be reduced.
There are ample replacements for AB, but these decks are forced to choose whether they are protecting from removal or bypassing blockers, they do not get the free-roll of both for a colorless mana.
Storm
Pyromancer's Ascension
Grapeshot
Pretty rough, but we still have some cards to enable (Past in Flames) and reward (Empty the Warrens, Kiln Fiend) playing lots of spells.
A similar, but discrete deck, Dragonstorm could possibly see a resurgence in a slower environment. Rite of Flame had always played a big role in any DS list, but we have got another 2->3 ritual since then.
Goblin Electromancer can allow a turn 3 kill with Dstorm though. T2 electromancer, T3 any two 2->3 rituals + Spirit Guide -> Dstorm for 2 hellkites and karthus, tyrant of Jund.
UW Resto
Snapcaster Mage
Geist of St Traft
Kitchen Finks
Interestingly, Finks is not as much a loss as I would have thought initially. Looking at some recent lists, Blade Splicer has been displacing Finks in some cases already.
While snappy is a hard hit, I think the deck has so much to gain from a slower environment that it should be able to evolve without too much effort. We're talking 7 slots for the top 8 deck from the last GP. More blade splicers, wall of omens, and some other card and you are probably good to go. Squadron Hawk perhaps?
Burn
Dark confidant from the Rb version. Otherwise no losses.
Gains:
---affinity will likely be evolving away from the hyper-aggressive deck it has been
---If control can come up to mitigate some of the combo, this can probably steal some percentages there.
---Ding dong, the Finks are dead, the wicked Finks are dead. Matyr is gone too!
Challenges
---Finks may be gone, but that doesn't mean lifegain is gone entirely. Obstinate Baloth, huntmaster of the Fells, and Thragtusk probably all get some play in this new world. As 4-5 drops, we might be able to race them.
---there are still some pretty absurd hate cards people can play if they want. That said, 95% of what people actually have for it will be lifegain and Leyline of Sanctity. With just those two, you may be able to work around it with the right plan.
Twin
Kiki Jiki
Yeah, its a hit, but not an unsurmountable one. After all it, was an amazing deck in standard and mini-extended with just splinter twin, which it still has.
Looking at the Twin deck at the GP, it only had 2 kiki anyway. It probably hurts to replace those, but you can probably find a suitable filler in about 5 minutes of searching.
BW Tokens
They don't lose a darn thing.
That said, in a lot of ways, this is the epitome of a fair deck that encourages people to play sweepers, that are likely to be good elsewhere as well. I doubt it would be able to rise to become a menace to the format.
Definitely consideration as the Level One of the format.
Aggro Loam
tarmogoyf
No idea what it replaces him with, but he is mostly just a fat butt for the deck. There should be some replacement that can cover that job.
Living End
Nothing lost.
Gains some value from an increase in creature decks as compared to combo decks. At the same time, it loses value to the likely increased values in counterspells in this environment.
Probably a fine role player.
Definitely consideration as the Level One of the format.
Control Decks
Snapcaster Mage
Unburial Rites
Snapcaster sure would have been nice, but I think its a price we can pay if it gets the environment where we need it for our other cards to be relevant.
Unburial Rites, I hesitate to list, as its not even really a control card, its an oversight with gifts ungiven.
Any Ramp decks that exist are going to be great matchups for you, instead of the absolutely awful match ups they are in current modern.
Having a mana leak on turn 4 against midrange matters again without bloodbraid elf making you feel awful.
This is obviously the one of the most exciting parts of this proposed format.
Teachings
Gifts
Cruel
All could be possibilities.
I think there is probably a Bant control list thats pretty good as well. Eternal Witness can probably do a pretty impressive Snapcaster impression.