See, I disagree on that, Solidus. Rehashing old weapons might be seen as lazy by newer players, and then it would cost the Dev's three times the work trying to implement a new one.
I would say, if you're going to do a Mk.II of any kind of weaponry, it should be rare. New weapons should come more often than rehashed old, though I certainly would not mind Mk. II weapons at all. Mk.II flak is a wonderful weapon, and a perfect addition to the ranks, but making Mk.II of other weapons would be difficult due to the nature of some of the weapons.
New weapons that have the same principle as existing weapons, but do it in a different way, would be a more viable, long term option. Hwacha gives high burst damage explosive/shatter. Make a new weapon that deals slow and steady explosive/burst. Same concept, different execution.