It can be cool to have an extension or a DLC where the users are able to send xeno virus (by sending a ship for example) to a planet in order to reduce the population before an invasion or in order to destroy the economy/production/search of the planet 
.
A solution to find a cure would be to redirect the production into a planetary project (like the search project).
It can be also nice to have the possibility to research new kind of plague or technologies that reduce the impact of those plagues.