Amethyst cards focus on magic abilities and effects, with perks like drawing extra cards or exerting opponent Glimmers
While you won't find Arbok's best cards in some of the most expensive Pokemon TCG sets, it has a few appearances in vintage expansions, including Expedition
Day/Night had done a good job of making it easy to remember what state the creatures were in, but everyone involved realized that it wasn't working
The first one that comes to mind is Klauth, Unrivaled Ancient, who will help generate even more mana
You'll only have at most 1-2 turns to change the outcome of the game
Discerning whether abilities or extra power and toughness are worth more is non-trivial, quite context-dependent, and likely will be the key to properly building decks based around this new take on the mechanic, as there are ways to use it to add more power to the battlefield