Don't Make Your Deck Worse
Sorcery: Contested Realm's card pool is deep. The Realm comprises 40 locations, 100 cards across multiple decks, helmed by dozens of Avatars that all request different aspects to maximize their power. Within that, however, is the trap of making your deck worse. Let me explain.
Filling out a 60-card Spellbook and 30-card Atlas might seem easy given that deep card pool, but it might trap or lure you into softening what makes your deck powerful in the first place.

I'll use the testing of my {Magician} list, which aims to sit atop {The Immortal Throne}, as an example. Even with a Spellbook of only 60 cards, absent an Atlas, the risk of diluting your deck is high.
Initial Draft of Magician Combo
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Here are the powerful aspects of the against any opposing Avatar:
The threat of {The Immortal Throne}
Since both players can contribute counters to {The Immortal Throne}, the opponent might change their behavior so that they don't scale it.
{Kythera Mechanism} + {13 Treasures of Britain}
The most powerful draw engine in the game at the time of this article's publication. It scales with more allied units in the Realm (e.g., {Plague of Frogs} in an incredible enabler)
{Kythera Mechanism} + {Chaoswish}
Access to literally any spell in the Curiosa catalog is arguably the most powerful thing the deck can do. I've lost many games simply by not having sufficiently studied all the potential spells available to me.
{Plague of Frogs} + {Standing Stones} + {Occult Ritual}
As far as I know, nothing else generates as much mana as this combination. This is typically a payoff for having churned through a ton of cards with the {Kythera Mechanism} and {13 Treasures of Britain} to have functionally infinite mana by looping {Plague of Frogs} and {Occult Ritual} with {Return to Nature} to combo out {The Immortal Throne}.
{Dhol Chants}
To a lesser extent, {Dhol Chants} enables a win when {The Immortal Throne} is on five counters. With {Kythera Mechanism}, {Dhol Chants} triggers {The Immortal Throne} to six counters. If {Chaoswish} is among the cards you can cast for free, then {Chaoswish} will trigger {The Immortal Throne} to seven counters, and then you can choose any [7]-mana spell (e.g., {Death Dealer}), which triggers {The Immortal Throne} to eight counters and ensures you are atop it alone to win the game.

Aside from other spells that attempt to function as additional copies of these pieces (e.g., {Highland Princess}, {Browse}, etc.), everything else is included in the deck because the minimum Spellbook size is 60 cards.
I've no doubt that the ceiling of {Magician}'s power is absurd. In my testing, however, I've found that the other ideas to fill in the rest of the Spellbook are hurdles. Here are some of the ideas I've tried:
{Croaking Swamp} + {Sunken Treasure} + a [2]-mana site on turn two
Turn one {Croaking Swamp} into turn two Tower or {City of Plenty} will let you surface the {Sunken Treasure} on turn two. If you wait until turn three, you can convert another {Sunken Treasure}.
{The Lady of the Lake} will let you rebuy a sacrificed {Sunken Treasure} to draw even more cards.
This has been the most consistent opening sequence I've committed to for the shell to get through early development.
{Highland Falconer} + {Peace Offering} + {Unlikely Alliance}
Since the goal is to always open with {Croaking Swamp}, the above combination enables a turn-two {Highland Falconer} to tutor for {Peace Offering}. A turn three Tower, {City of Plenty}, or {City of Glass} can cast an {Unlikely Alliance} for three cards.
This is an attractive sequence, but there are some fail-rates built in:
{Highland Falconer}'s other available targets aren't good
{Peace Offering} never survives the turn to actually make the offer. As a result, {Peace Offering} demands a movement site like {River Rapids} to push it immediately onto an opposing site, which dilutes the already tight space available for sites.
This might seem reasonable, but {River Rapids} does nothing to support any of the aforementioned power-level reasons to play the deck. As a result, it dilutes the shell since when it does what it's in the deck to do, it's still not advancing you toward the reasons you're playing the deck.
I've considered including {Palliburie Bats} for {Highland Falconer} to tutor, so I can also play {Buried Treasure} to further lean into the {Sunken Treasure} engine. {The Lady of the Lake} and {Dozmary Pool} can play a {Buried Treasure} underwater for Frogs to carry to the surface as well.
{Palliburie Bats} can attack a {Peace Offering} that was given to the opponent, which can be {Return to Nature}d and then tutored for again by {Highland Falconer}. Since {Highland Falconer} is a mortal, {The Lady of the Lake} can reanimate it as well.

{Demonic Contract} + Unique Demons
Since all the combo pieces that comprise the powerful aspects of the deck, any non-Unique demon will not be sufficient for why you would include {Demonic Contract}
Unique demons include {Lilith}, {Cerberus in Chains}, {Mephistopholes}, {Asmodeus}, and {Balor of the Evil Eye}
Aside from {Cerberus in Chains}, which is the cheapest to cast and is a thick defender, which is desirable, the other demons do nothing to advance you toward what the deck wants to do.
Despite being individually powerful, they will be less individually powerful in a deck designed to assemble a combination of cards rather than a deck full of raw-rate powerful cards.
You can see how one idea can spiral into another and another and so on until you forget what the deck even aims to do.
Work in Progress
I know many creative minds are also brewing concoctions of their Avatar of choice. When you feel stuck, rewrite your decklist, starting with what the powerful aspect aims to accomplish. Include only those cards, then try to include cards that support that plan in some way.
Of course, every deck likely needs to include its vegetables, like removal or meaningful disruption. There may be vegetables that can simultaneously move you toward your goal rather than solely serving as filler.
Cheers to those who suffer the slings and arrows of losses in pursuit of the grander experiment that is pushing your Avatar of choice up the tier list.
I'll continue iterating on {Magician} and hopefully discover yet-uncovered gems to share in the future.
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Featured image: {Dhol Chants} by Dan Seagrave