Sideboard Theory:

>Nykthos Ramp:

In: 3x Damping Sphere, 2x Inscription of Abundance
Out: 4x Werewolf Pack Leader, 1x Briarbridge Tracker.

Damping Sphere is basically required to stop Nykthos, Shrine to Nyx. Bite Down allows us to kill their dorks early, and will force a trade with most creatures if they block.
We want every creature to be able to trade with their inevitable Old-Growth Troll, so Werewolf Pack Leader is out as a 3/3. Briarbridge won't survive a Cavalier after being buffed by Vivien, Arkbow Ranger or Inscription, so they're less valuable in this matchup.

>Rakdos Midrange:

In: 3x Nullhide Ferox.
Out: 2x Aspect of Hydra, 1x Steel-Leaf Champion

Ferox means that any discard from Liliana of the Veil, or Kroxa, Titan of Death's Hunger gives us a hexproof 6/6.
Removal means devotion won't be too high so Aspect is out, Steel-Leaf can't outgrow a 4/x and leaves no value when it dies.
Brairbridge is the other option to cut, it doesn't survive as many combats, but is a 2 for 1 regardless thanks to the clue, the vigilance also allows it to block the Fable of the Mirror-Breaker goblin token when it attacks even if I swing out.

>Greasefang:

In: 2x Scavenging Ooze
Out: 2x Briarbridge Tracker

Ooze can remove important graveyard cards.
Briarbridge comes out as vigilance is irrelevant against fliers from Parhelion.
The goal here is to just be faster.

>Azorius Control:

In: 3x Tamiyo's Safekeeping
Out: 2x Yorvo, Lord of Garenbrig, 1x Briarbridge Tracker

Safekeeping helps dodge removal and wraths.
Yorvo, getting big with no evasion doesn't help here, Briarbridge is the least useful three drop.
The game comes down to choosing the correct time to CoCo and out-pacing them.

>Mono-White Humans:

In: 3x Inscription of Abundance
Out: 1x Garruk's Harbinger, 1x Vivien, Arkbow Ranger, 1x Briarbridge Tracker

Using Inscription to fight Brutal Cathar and Containment Priest is the way to go here, just keep up pressure and force them to sacrifice their creatures to block.
Cut one Vivien as this matchup will rely on speed, down one Harbinger and Tracker as their creatures will grow to 3/x and be in tradeing range quickly.