My entry would be a game mode called The Plague. In this mode, one Splinter each season will be designated as Plagued. Archons from a plagued splinter has a chance to apply a random debuff or at a much lower probability a random buff to its units. Winning matches using a Plagued Archon gives +20% reward bonus.

Core Mechanics
• Base Format: Modern
• Plagued Splinter: One Splinter is designated as Plagued each season.
• Trigger: The plague only activates if you summon with an Archon from the Plagued Splinter.
• Using any other archon = completely normal battle (no plague risk).
• Plague Effect (rolled once at the start of battle, applied to all your units only):
• 70% chance – Debuff: Your team is struck by the plague. All your units suffer a random negative effect (e.g., reduced Health/Attack/Speed, Poison, Burn, Slow, etc).
• 10% chance – Mutation: The plague mutates your forces positively. All your units gain a strong random beneficial effect (e.g., increased stats, Enrage, lifesteal, Dodge etc.).
• 20% chance – Cleansing: The Gods have granted you the gift of Cleansing. The plague passes without effect, and the battle proceeds completely normally.

In this mode, a Plagued Splinter will always be available every match.