Altered Movement
Status Effects (buffs, debuffs, conditions)
Wounding
Damage Mitigation (armor, shields, evasion, stealth, incorporeal forms)
Rest/Recovery
Turn Order
Skill Usage / Skill Mini-Games / Group Skill Challenges
Battlefield Tactics (cover, concealment, terrain, weather)
Consumable Tracking (ammo, money, potions, durability, survival elements)
Equipment Systems (gear, crafting, or even base-building)
Special Abilities (powers, feats, spells, psionics, teamwork actions)
Item Crafting / Enchanting / Base Building
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Investigation Systems (dedicated mechanics for mystery or discovery play)
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Social Dynamics (factions, relationships, reputation)
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Random Generators (encounters, NPCs, hooks, tables)
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Anything else that might apply to your game.