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