Good examples of echo are Mogg War Marshal and Stingscourger, where you have to pay the echo cost to keep the creature one turn after youve cast it
You could consider Fossil Cards as placeholders, if worst comes to worst
Having access to repeatable cheap GY hate was crucial during longer games, where the graveyard was the waiting room between board wipes
(1) Special Summon 1 Level 4 or lower Fish, Sea Serpent or Aqua monster from your hand or GY, and if you do, you cannot Special Summon from the Extra Deck while you control it, except Xyz Monsters
Sway of the Stars does bring annoying big-meme energy to casual Commander rather than spicy options for cEDH, but it feels safe in a world where Worldfire got unbanned without setting the format aflame
A lot of black cards will emphasise the sacrifice element which will come in the form of direct downsides to your cards such as you having to lose a creature or pay life to achieve something