5D's: This is most evident when comparing the two visually: both Dark Synchro and Xyz Monsters are black cards with a left-aligned row of modified star pips to represent an alternate property to standard Levels: Negative Levels and Ranks, respectively
What makes the mechanic different this time around is that the Pokemon can be more than just dual-Darkness or dual-Metal (like Fire / Water)
Your Extra Deck should consist of Xyz, Synchro, Fusion, or Link Monsters that complement your Main Deck strategy, while your Side Deck should contain cards that can counter specific threats or strategies you might face in a match
Synchro Monsters have some of the best support cards, allowing for easy Synchro plays to bring out powerful Boss Monsters
If SPYRAL is able to go off undisrupted, there is little chance of breaking this opening board
The best Elf commanders in MTG are the heart of your deck