Sideboard Theory:

> R/B Midrange:

Their Problem Cards:
Bonecrusher Giant - Stomps our mana dorks when we're on the draw, can trade with most of our creatures.
Graveyard Trespasser - Can trade with most of our creatures when it flips.
Sheoldred, the Apocalypse - Can trade with all of our creatures, deathtouch makes it almost impossible to do better than 1 for 1.
Fatal Push - With Revolt active will kill next to everything in the deck
Extinction Event/Ritual of Soot - Will almost certainly decimate our board, very bad if they land it and we're out of gas.

In:
3x Go for the Throat - Lets us dictate the pace of the game and stops them snowballing value.
2x Shapers' Sanctuary - Makes their 1 for 1 removal significantly worse, allows us to keep grinding into late game.
2x Garruk's Harbinger - Draws cards and is hexproof, powerful tool.

Out:
2x Llanowar Elves - In this removal heavy match elves are less important, shaved two.
2x Yorvo, Lord of Garenbrig - No evasion or value when it dies.
2x Ghalta, Primal Hunger - Easily removed and hard to cast with limited resources.
1x Quirion Beastcaller - Takes a while to grow to an important size.

Pressure is the key here. Deny them their 2 for 1s, try to make your own.

> Nykthos Ramp:

Their Problem Cards:
Cavalier of Thorns - A 5/6 will block all of our non-buffed creatures.
Karn, the Great Creator - Enables the combo, can fetch silver bullets like The Mightstone and Weakstone and Cityscape Leveler.

In:
3x Go for the Throat - Removes their creatures if they start to establish board, lets us get through to their planeswalkers.

Out:
3x Werewolf Pack Leader - Almost nothing in their deck is smaller than a 4/4.

Opening with an elf is key, especially since we've cut almost all our two drops, don't be afraid to mulligan for a fast opener.

> Gruul Boat

Their Problem Cards:
Bonecrusher Giant - Stomps our mana dorks when we're on the draw, can trade with most of our creatures.
Skysovereign, Consul Flagship - A flying 6/5 is difficult to remove and hard to race, will most likely need to be fought or forced to block.
The Akroan War - Steals our biggest threat and hits us with it.
Obliterating Bolt - Kills half of our creatures.

In:
3x Cankerbloom - Hitting all their big non-creature spells is great, being able to be hit off CoCo is also nice. Can be sacrificed in response to a Stomp so it doesn't resolve.

Out:
2x Quirion Beastcaller - Can be Stomped if we're not careful, but not a terrible card in the matchup, shaved two.
1x Steel Leaf Champion - Doesn't get any bigger and the evasion doesn't always help.

> G/W Angels

Their Problem Cards:
Almost all their creatures have flying and a lot have vigilance so blocking/attacking becomes tricky.
Skyclave Apparition - Removes any creature.
Bishop of Wings - A 1/4 for WW is very strong, especially when it enables powerful synergies, we need to get out the gates early and force them to chump with it.

In:
2x Skyreaping - This on turns 4-5 should be enough to clear their entire board and set us up for the win.

Out:
2x Yorvo, Lord of Garenbrig - Just slightly worse than the other creatures, not bad in the matchup, but something had to be trimmed.

> Lotus Combo

Their Problem Cards:
Emergent Ultimatum - Tutors for their combo
Languish - Kills almost all of our creatures.
Ritual of Soot - Kills almost all of our creatures

In:
3x Necromentia - Turn 2 Necromentia is one of the most powerful things you can do in the format. If the manabase allows...

Out
3x Werewolf Pack Leader - A little too slow, hoping not to have to rely on our two drops.

No creatures and no removal, a classic combo vs aggro race.

> U/W Control

Their Problem Cards:
Fateful Absence - Efficient removal.
Supreme Verdict - Kills all of our creatures
Farewell - Kills all of our creatures
Aether Gust - Efficient removal.

In:
No Changes

Out:
No Changes

>Mono-White Humans

Their Problem Cards:
Brutal Cathar - Repeatable removal.
Skyclave Apparition - Removes all creatures
Declaration in Stone - Efficient removal.
Destroy Evil - Efficient removal.

In:
3x Go for the Throat - Destroys a Brutal Cathar and gets our creatures back.

Out:
3x Werewolf Pack Leader - Doesn't race quite as hard as we'd like here.

>Abzan Greasefang

Their Problem Cards:
Greasefang, Okiba Boss - Enables their combo, kill on sight.
Abrupt Decay - Efficient removal.

In:
3x Cankerbloom - Removes their vehicles at instant speed, worst case a 2 mana 3/2.
3x Infernal Grasp - Removes Greasefang and stops their combo.

Out:
4x Werewolf Pack Leader - Too slow in this matchup, need more pressure early.
2x Quirion Beastcaller - A little slow, but has the potential to grow and put on pressure.

>Izzet Phoenix:

Their Problem Cards:
Thing in the Ice - Can force us to return all our creatures to our hand if it flips.
Arclight Phoenix - Recurable flier makes for an inevitable clock
Lightning Axe - Will kill most of our creatures.

In:
2x Skyreaping - Kills Ledger Shredder and Phoenix.

Out:
2x Quirion Beastcaller - Easiest creature to kill, not bad in the matchup, but had to make room somewhere.

>Five-Colour Fires:

Their Problem Cards:
Fires of Invention - The engine of the deck.
Chained to the Rocks - Cheap removal.
Leyline Binding - Cheap removal.

In:
3x Cankerbloom - Having to lose board presence to slow them down isn't as good as repeatable removal, but in a pinch will save us.

Out:
2x Yorvo, Lord of Garenbrig - Not as valuable as any of the other creatures in the matchup.
1x Quirion Beastcaller - Doesn't pose enough of a threat early on against bigger creatures. Shaved one.