For once, a serious post to get us back on topic. Chris mentioned in an interview that it should be easy to guess because the next one will be based on the pattern they've followed so far. And he said he started it right after Uplink, but stopped to make Darwinia and Defcon first. As for the pattern, if any of you have played Uplink, you noticed that among all of the different companies, there was Darwin, who was studying AI through a complex simulation of life (aka. Darwinia). Also, there was another server that was basically Defcon (an easter egg; you couldn't actually do anything with it), and the password to access the server was "Joshua" (Wargames). The pattern I see is that in each game, you're still the uplink agent.
SPOILER ALERT - Darwinia
If you'll remember, at the beginning of Darwinia, Dr. Sepulveda asks you who the heck you are and how did you get here, so you must have hacked in. And at the end, it turns out that Darwinia was attacked by a virus (or two, I forget). My guess is that you're the uplink agent hacking into the Darwin server to clean out the virus.
As for Defcon, there's no real evidence that you're playing a hacker, but it's based off Wargames and it's just as connected to Uplink as Darwinia. Therefore, I predict that Updefwinia will either be based off another server in Uplink, or will in some other way be related to the game. If the rumor is true that updefwinia will bring all the games together, then the only way I can see it happening is if it's Uplink 2. And if that's the case, I definitely see level 3 brains as a possibility. But goodness me, what would a hacker do with a tassel of doom?