As always, I adamantly support the possibility of modding, as well as the possibility of defining the universe exactly as you want. (So you could fight the sci-fi beasts from some particular sci-fi universe without having random things thrown in there as well.) So my hope would be the ability to design creatures and have them as blueprints that the server could call and spawn based on how you've defined it. And as for importing your own models, of course.
For ingame, I'd prefer customizing creatures to require DNA manipulation and trial-and-error, where you can't be sure what you're going to get. But I suppose that same editor should be thrown ingame as well. Not only would it be useful for testing, but it'd be what people would want on some servers or for admins, and I'd rather not force people to mod in something that can be added directly by the development team.
No promises on details as always, but that's my initial reaction.
Now viruses... interesting concept. More as debuffs to people more or less, like interfering with their vision or slowing them down... though of course it could be taken further than that. Something I'll have to think about. Also, always fun to be able to trip hardcore in games, so maybe you could make a virus for your own recreation that causes everything to get colorful and wave back and forth. :D I love things like that in games, haha.
