[Aris_Sung]

In addition to what I added before, how about plagues that can be research that can kill only a specific species. To do this, each species can have a genetic code that is kept secret and parts of it can be stolen at a time as an intellligence mission. That way you can make plagues that are deadly to other races but which you are resistant to. So you could plague bomb or intel mission release plague on a planet of aliens, when they're dead you can claim it and not have to 'cure' the planet.