Disable heavy weapons during invades. Will it make it harder for people to kill invader or the invader kill the defenders, yes. But its a way to change the gamemode without nerfing anything. Edit: One idea I've seen was during the invade you got a weapon damage boost. Not enough to make snipers a one shot to the body, but enough that you can still do enough damage to the boss. (maybe don't even include snipers, shotguns, sidearms, and fusions.)