The spoiler season for Dominaria United included a teaser list posted by Maro on his Tumblr account. The clues is a return of sort of a classic spoiler method back in the days of Inquisition Magazine published whenever there is an upcoming Magic: the Gathering set.
I have followed this Reddit thread of this updates as of August 21st, 2022 and here are the hints posted below.
A New Evergreen Counter – Stun Counters. It reads as “if a permanent with one would become untapped, a stun counter is removed from it instead.”
A Popular Planeswalker Reprint – Liliana of the Veil
A Blue Card with “Destroy All Creatures.” – The Phasing of Zhalfir
A Five Mana Value Creature that can be a 10/10 – Territorial Maro
An ability word that didn’t start as an ability word returns – Domain. It first appeared in Invasion before becoming an ability word in Conflux.
A Card that Grants Poison – Ajani, Sleeper Agent with his Ultimate ability granting an emblem that gives an opponent two poison counters whenever you cast a creature or planeswalker spell.
A Character players have been requesting get a card for years finally gets one – The Raven Man
“You may Exile an Instant or Sorcery card with mana value X or less from among them.” – Keldon Flamesage.
“Enchantment creatures you control have.” – Zur, Eternal Schemer
“For each opponent who doesn’t, that player loses 2 life and you draw a card.” – Braids, Arisen Nightmare. At the beginning of your end step, you sacrifice a permanent, then each opponent may do the same with a permanent of the same type. If they don’t, the above happens.
Three Original Weatherlight Crew Members are in the set – Squee, Karn and Ertai (no card reveal yet).
“Pay any amount of mana.” (Karn, Living Legacy, his -1 lets you do this, then look at the top X cards of your library, putting one into your hand and the rest on the bottom).
“Vehicles you control have crew 1.” (Astor, Bearer of Blades, also gives Equipment equip 1; relevant as you can get a Vehicle or an Equipment from your top 7 into your hand with his ETB)
Legendary Creature – Elemental Demon (Solkanar, the Tainted)
FYI, the original post of the Maro clues is located here: https://markrosewater.tumblr.com/post/692666879432622080/maros-dominaria-united-teaser