Hyper Velocity Acid Cannons, or regular HVC's are a must have in every si-fi universe. They are my favorite future space weapons and would love to see it as a improvement.
It would be really cool to see the long barrel of a HVC on a planet and give a really sick bonus like -10 hp to enemy ships or +20% for planetary defense or -x% to a enemies invasion method.
Even more awesome of a idea is a improvement that takes part in orbital combat. Like the HVC could do damage per round like a ship.