Confidence:StrongSolidPlausibleTentativeSpeculative

• Innkeeper's Talent

  1G

  Enchantment — ClassG

  Counter synergy: both cards work with the same kind of counters, so their effects stack in a counters deck. Not necessarily a closed loop on its own.
• Mirror Room // Fractured Realm

  2U5UU

  Enchantment — Room // Enchantment — RoomU

  Mirror Room makes Parallax Nexus's triggered ability trigger an additional time — each time it fires, you get it twice. Value, not a loop.
• Rings of Brighthearth

  3

  ArtifactC

  Parallax Nexus has activated abilities that aren't mana abilities, fueling Rings of Brighthearth's activation-trigger.