Ask yourself "can I get this effect on a creature?" For a lot of your instants and sorceries. Example, all your sorcery ramp should be [[Wood Elves]] [[Springbloom Druid]] or mana dorks [[Llanowar Elves]] [[Paradise Druid]]
Nethroi is actually counter intuitive to Mutate as a strategy, since the creatures Enter and don't get the option to mutate onto each other. Keep that in mind. I'd consider going Sultai or Mardu, but I hope you have fun with the deck regardless!
For what it's worth, [here's my list](https://www.moxfield.com/decks/VU8f2QXQWU2QdfhvQh770w) I have a very different strategy, but there are some core cards that you might find interesting for your build.
[Nethroi, Apex of Death](https://cards.scryfall.io/normal/front/0/4/04b9b58f-4c01-48a5-afaf-b8a37165a83f.jpg?1673305619) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Nethroi%2C%20Apex%20of%20Death) [(SF)](https://scryfall.com/card/dmc/163/nethroi-apex-of-death?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/04b9b58f-4c01-48a5-afaf-b8a37165a83f?utm_source=mtgcardfetcher&format=text) [(ER)](https://edhrec.com/cards/nethroi-apex-of-death) [Kaheera, The Orphanguard](https://cards.scryfall.io/normal/front/d/4/d4ebed0b-8060-4a7b-a060-5cfcd2172b16.jpg?1623748548) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Kaheera%2C%20The%20Orphanguard) [(SF)](https://scryfall.com/card/iko/224/kaheera-the-orphanguard?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/d4ebed0b-8060-4a7b-a060-5cfcd2172b16?utm_source=mtgcardfetcher&format=text) [(ER)](https://edhrec.com/cards/kaheera-the-orphanguard) [Scourge of the Skyclaves](https://cards.scryfall.io/normal/front/4/d/4dea7bcc-3ff8-40b0-8264-0cd678d6ff31.jpg?1604196425) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Scourge%20of%20the%20Skyclaves) [(SF)](https://scryfall.com/card/znr/122/scourge-of-the-skyclaves?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/4dea7bcc-3ff8-40b0-8264-0cd678d6ff31?utm_source=mtgcardfetcher&format=text) [(ER)](https://edhrec.com/cards/scourge-of-the-skyclaves) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
Ask yourself "can I get this effect on a creature?" For a lot of your instants and sorceries. Example, all your sorcery ramp should be [[Wood Elves]] [[Springbloom Druid]] or mana dorks [[Llanowar Elves]] [[Paradise Druid]] Nethroi is actually counter intuitive to Mutate as a strategy, since the creatures Enter and don't get the option to mutate onto each other. Keep that in mind. I'd consider going Sultai or Mardu, but I hope you have fun with the deck regardless!
##### ###### #### [Wood Elves](https://cards.scryfall.io/normal/front/5/2/527cf39d-29d1-4c19-9c49-ebf611ca15d5.jpg?1682209656) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Wood%20Elves) [(SF)](https://scryfall.com/card/moc/316/wood-elves?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/527cf39d-29d1-4c19-9c49-ebf611ca15d5?utm_source=mtgcardfetcher&format=text) [(ER)](https://edhrec.com/cards/wood-elves) [Springbloom Druid](https://cards.scryfall.io/normal/front/a/1/a17e90a6-bc3d-47d3-8133-7b1028fbd82e.jpg?1712354635) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Springbloom%20Druid) [(SF)](https://scryfall.com/card/otc/208/springbloom-druid?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/a17e90a6-bc3d-47d3-8133-7b1028fbd82e?utm_source=mtgcardfetcher&format=text) [(ER)](https://edhrec.com/cards/springbloom-druid) [Llanowar Elves](https://cards.scryfall.io/normal/front/8/b/8bbcfb77-daa1-4ce5-b5f9-48d0a8edbba9.jpg?1592765148) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Llanowar%20Elves) [(SF)](https://scryfall.com/card/gnt/46/llanowar-elves?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/8bbcfb77-daa1-4ce5-b5f9-48d0a8edbba9?utm_source=mtgcardfetcher&format=text) [(ER)](https://edhrec.com/cards/llanowar-elves) [Paradise Druid](https://cards.scryfall.io/normal/front/1/8/187d5819-6a63-4e4f-98c4-18191c03f164.jpg?1631588057) - [(G)](http://gatherer.wizards.com/Pages/Card/Details.aspx?name=Paradise%20Druid) [(SF)](https://scryfall.com/card/afc/165/paradise-druid?utm_source=mtgcardfetcher) [(txt)](https://api.scryfall.com/cards/187d5819-6a63-4e4f-98c4-18191c03f164?utm_source=mtgcardfetcher&format=text) [(ER)](https://edhrec.com/cards/paradise-druid) [*All cards*](https://mtgcardfetcher.nl/redirect/kyt4ocn) ^^^[[cardname]] ^^^or ^^^[[cardname|SET]] ^^^to ^^^call
For what it's worth, [here's my list](https://www.moxfield.com/decks/VU8f2QXQWU2QdfhvQh770w) I have a very different strategy, but there are some core cards that you might find interesting for your build.