A unique military ability that applies to certain troops, starting in the early modern era, such as hurling grenades that make a tile in a battle do damage to any troops standing on it, could change the way battles are fought, as certain tiles are made inhospitable and players must decide that sacrificing a troop is really worth killing an enemy soldier.

If it is seen as too powerful, it could be limited by army, such as only 1 chemical trooper per army, or each chemical trooper have only 1 grenade per battle.

It could also be cool to add it in the civics, such as deciding if you're for or against the chemical weapons, for means you can now build them, against allows Grievances against enemy nations who do allow them or when they're used against you on the field of battle.