Mostly a classic Bant Eldrazi list, testing out a few different flex slots.
Thalia, Guardian of Thraben does a few nice things for the deck. For starters, it's a top-notch hoser for a lot of common decks in modern, and the disruption that it provides is very relevant since it gives us a way to interact with spell-based decks pre-sideboard where normally we'd be pretty cold to something like storm game 1. Additionally, it has minimal impact on us since our noncreature spells are either 1 mana (path, stirrings) or are mechanically unaffected by Thalia (Engineered Explosives). On top of all of these things Thalia provides us with a 2-drop; generally hands without some mana acceleration (temple, hierarch) are unkeepable, but having some number of proactive 2-drops to bridge the gap in games without acceleration help reduce how much the deck needs to mulligan.
Walking Ballista Was originally a sideboard card that quickly proved itself worth a maindeck slot. It only requires 1 slot in the 75 thanks to Ancient Stirrings being able to find it, and in the matchups that it's useful in it's so potent it can often win games on its own. It's exceptionally strong against CoCo and Noble Hierarch decks, which incidentally are also often very light on removal but can be difficult to push damage through against because of how wide their boards tend to get. Some additional creature interaction in these matchups is also important since they often have a combo finish or other ways to go over the top that depend on a number of low-toughness creatures.
Gavony Township is a land that I'm surprised rarely sees play in Bant Eldrazi, honestly (though I'd also never tried it myself in the deck). Being able to get double-value putting double tokens on Skyspawner + token, or spare Noble Hierarchs, as well as synergizing with Walking Ballista all give it a useful niche. This is the flex slot that I'm least certain about; there's a limited amount of space for extra mono-colorless land and a wealth of strong options including Ghost Quarter, Scavenger Grounds, Gemstone Caverns, and Sea Gate Wreckage.
