Aside from Standard, Pioneer, and Limited, here's what's exclusive to Arena: Alchemy: This is essentially standard
Lately, the spotlight has swung back to Gen 7 and older sets, with collectors and investors driving a full-blown revival of XY-P promos and iconic Pikachu singles
Your party consists of up to one cleric, rogue, warrior, and wizard
Add to that the possibility of destroying some (or all) of the creatures on the board and suddenly Rakdos becomes a lot better
Historically, Stage 2 Pokmon (Other than Gardevoir and Charizard, who had its own acceleration mechanic) had really struggled to keep up with the game but adding a single copy of Rosa can truly make the difference in decks like Garchomp ex or Dragapult ex, allowing them to have a much stronger late game