Thousand-Year Elixir doesn't actually grant haste to creatures you control, nor does it let you attack with them as though they had haste
Being Black and Green gives you access to all of them, combined with strong abilities
Illustrated by Gabor Szikszai Standard Not Legal Alchemy Not Legal Pioneer Not Legal Historic Not Legal Modern Legal Brawl Not Legal Legacy Legal Comp
Nothing in the deck prohibits players from casting spells, and indeed, I even run Helm of Awakening to encourage players to do so
you can still play cards like Black Lotus and the rest of the Power Nine, but only one copy of each
The Standard meta is going to get shook up something fierce with the release of Origins, so anything could happen, but if Nissa pops up at top tables in some early Standard tournaments, I can easily see her pushing the upper limits of standard playable mythics