Value

There is not, strictly, a theme--I simply brute force good numbers. I churn out a lot of card advantage through DS-Genesis. I operate on the premise that the player who spends the most mana usually wins the game; my Avatar wants to stand on position 3/18 the entire game, so I don't waste a single Site placement--this is also why I don't always run Ring of Morrigan. At its heart, this is a midrange deck:

2:1s that don't set me too far behind
Enough life gain to survive the aggro lists while I turn the corner with,
High-rate creatures to maximize virtual card advantage
Curve that stops at 7 for high-scaling-inevitability
Utility lands that either slow your opponent, or create more creatures

Games where you open with Geistwood and make your Genesis creatures read from the Pnakotic Manuscript are, usually, one-sided. I don't mulligan looking for uniques--because we have strict thresholds to meet--but, sometimes, you get lucky.

Site Selection

We hope to play a land every turn. Land Surveyors & all sheer cards drawn make this much easier. The more area we intend to cover, the more we need to defend. I try to fit as many Bridges, Paths, Copses, etc., as I can, based upon the meta I expect (my LGS runs a lot of water).

Threats

Take only the best Genesis creatures. There are more available than you can possibly fit, so don't be tricked by the keyword where there are better options. Make them "sticky!"

The Hexham Haunts & Crown Prince (18 other mortals) keep coming back
Lady Iseult for infinite Genesis
Young Master Damion with immortal bodyguards, and two more for the road
Talamh Dreig with immunity to the forced-movement water menaces (also, Bury & Drown)

Answers

Avoiding cards that can get stuck in your hand vs. playing what is most efficient. I lean into consistency.

Lightning Bolts that have no targeting restrictions and a floor of damaging Avatars
Thunderstorm for similar reasons--though I often hope it deals 9 damage to Avatars
Bury & Cast Into Exile as emergency removal. I limit Exile to one copy, as it does have a restriction
Earthquakes that have no targeting restrictions and ignore Ward
Craterize where we are more likely to get value from dead creatures and often have more sites than we can use

Packages

Assorted Animals & Falconer: [Grain Sparrow, Peace Offering, Palliburie, Vampire Bats], Root Spider, White Hart, Black Cat, Gargantula, Conqueror Worm
Returned King: Boulevard, Master Necromancer, Damion, Draconian Bonekite
King of the Realm: ~20 mortals, but I tend to avoid this King because he dies too easily for 7 mana
Anti-Aggro: Perilous Bridge, Pebbled Path, Holy Ground (I'm low on this one), Kissers of Wounds, Grain Sparrow, Ring of Morrigan

Threshold & Mulligans

Gothic's powerful additions come at a steep cost. As much as I love Vile Imps, I don't feel Fire Geneses scale as well as Air, or Earth. Rather than waste precious Site-equity, I cut fire. Archangel Michael is worth it, but I do miss Sir Morien.

I tend to keep four or five cards. I aim for one Earth and one Air site, and the third site is usually kept or mulled based upon the spells I'm keeping. It may surprise you that I prefer not to have Cores in my opener; I'm more likely to keep them, on the draw. They often empty my hand before I have anything else to do with DS. The exception is usually Grandmaster Wizard, which I warp my play around casting asap.

See also: (https://stattrek.com/online-calculator/hypergeometric)

Collection

I have so many mana sinks, that the collection is almost wasted. 7 slots are taken by Ghouls and Knights, and the other 3 slots are a distraction for Legions of Gall. If you find you require more answers than threats in your metagame, cut a few Gifts of the Wolf to make room for Toolboxes.

Meta

The Maybeboard is about your metagame. My list is what I would consider 'stock,' for threats and answers. If you play against a lot of Mortals, or Undead, bring in the relevant King. Up against Savior? Bane of Aventis likely turns the board around. As for what I'd call the flex-slots, I'd swap out top-end threats:

Adtonitum, Talahm Dreig --> Kings if you play with/against Undead and/or Mortals
Paladins of Bazia --> Bane of Aventis
Forlorn Keep --> Impenetrable Copse, Pebbled Path, Holy Ground, Haystack, Boulevard of Bones, etc.
Cloud City, Merlin's Tower --> Perilous Bridge, Fields of Phyxis, Mountain Pass

Afterlife 60/30

@Vorlind