I think that there should be multiple types of digital viruses,each with different objectives and upgrade trees (symptoms?).I have a few examples:
1.Trojan virus.
The main objective for this virus is to infect and control every electronic device on the planet.Trojan virus is basically the same as the neurax worm in the game,for the computers. It will start with basic infection methods (throught ads,corrupted files,etc,etc). The stronger (better antivirus breaching,wi-fi filiter bypassing) the virus gets,the more abilities it unlocks based on the devices it infected. The abilities will be listed on a sidebar,similar to the necroa virus (much bigger one,thought,so it may be a good idea to make it expandable).These will allow you to perform different actions in a certain country,such as shutdown every computer in the country for a period of time,etc.
The more trojan evolves,the more abilities it gets. In the end,it'll be able to launch nukes and raise defcon levels at will.
2.Extermination virus.
The extermination virus works differently from the trojan virus,the main difference being the removal of all passive abilities and an objective to destroy every device in the world. It'll start,again with different types of breaching/infection abilities,but they will be a bit different from trojan's (but more effective).However,the latter upgrades allow the virus to destroy/shut down the devices in alot of different ways,be it melting the core,making the computer explode-any way you can think of.The difference from trojan,is,however,that the abilities will not be activated at will. So choose your upgrades carefully. While you won't be able to assume direct control for this virus,the latter upgrades allow you to destroy nuclear reactors/take down sattelites/etc.
3.Hybrid virus (can't think of a different name,sorry).
This virus is an interesting hybrid between the original plaugue inc. viruses and digital viruses.The goal for this one is to destroy
humanity (or enslave,if possible).The virus,unlike the previous two,will spread with the machines,but the humans who make contact with the devices will be infected with a standart,human-hazardrous virus.It starts out in a similar fashion to the other two. However,it will have two separate symptom/upgrade trees. The first one is a standart digital virus upgrade tree,however,all of these upgrades are mostly focused on infectivity. The main upgrade-an ability to transfer the infection to humans-will also unlock different types of said transfer,from wi-fi waves to simply touching the device.The second tree,however,is a standart plague inc. symptom tree,and will be available as soon as you unlock the transfer upgrade. It has it's differences,and the main one is that most of the upgrades increase severity/lethality for human beings.I also think that the player should get different choices,similar to the neurax worm (enslave/eradicate humanity,make everyone a psychopath,etc.)
I really think it'll be good for the digital virus to be released as a free expansion pack,simply because of the sheer amount of content it will have.
. thanks everyone.