I think that in designing a 4E science fiction idea, I’d have to ask, “what is a class”.
In D20, a class is essentially a cluster of abilities that can be tacked on to a character, even if other people think it is kind of tacky.
In 4E, classes seem to be more of a metaphysical limitation. Thinking about how I would do a Flash Gordon style campaign in 4E, for example, I think I would not want classes, because why couldn’t Dale Arden or Doctor Zarkov learn to do what Flash Gordon does? In a 4E game you don’t want one superhero and four supporting cheerleaders: you want five superheroes.
Something like my Third Legion idea, which is planned for BESM D20, classes might be apropos because it is based on the idea that advanced technology allows anybody who wants to to become a superhero. Most people are not going to make the investment, but for those who do, there would be Cyborgs, Intellects, Mystics, Strongmen, etc. Restrictive classes like 4E has would be saying that the investment to really master cybernetic attachments precludes learning advanced engineering or mystical arts, except for in special cases.
Another possibility that comes to mind is to define the races in a science fiction setting as classes. Since 3E came out, there’s been the idea that any race can do anything, but if you take a more scientifically-oriented view of things, some things will just be a lot harder or even impossible for members of other species. An example is in Jean Auel’s Clan of the Cave Bears, where the neanderthals just could not duplicate Ayla’s proficiency with the sling because their arms would not swing in that manner. “Multiclassing” would only be available for hybrids.
Some ideas for converting common science fiction settings to 4E:
Star Trek: Classes would be Command, Security, Medical and Engineering. Multiclassing would be allowed. Anybody would be allowed to take powers relating to ship operations, such as helm manuevers or firing patterns. Command could be based on the Warlord class, while Security could be based on the Fighter and Ranger.
Star Wars: Classes would be Jedi, Scoundrel and Noble. Anybody could learn technological, combat or medical skills, so anyone could jump into a gun turret or fly an X-Wing or try to use a medical kit.
Flash Gordon: Either go classless, or use races as classes. Characters differentiate themselves according to the powers and feats they choose, but all options are available for all PCs.
Battlestar Galactica: While one could define PCs as medics, command, scoundrels, etc, I think it is probably best to make them all one type, such as Colonial Warriors. Within that type, PCs can specialize for different roles by choosing different skills, feats and powers. As Colonial Warriors, I think they have the maximum opportunity for interesting adventures, and it would probably be easier than trying to fit in a Medic character all the time, or figuring out how to keep the Command characters occupied.
Babylon 5: Again, I think it is best to go classless. Medic and scoundrel type characters are easier to work in than they are in BG, but I think without a compelling reason, I don’t see a need to define restrictions on what characters can do. Perhaps specialization could be handled as Paragon Paths, e.g. Ranger, Doctor, etc.
It is interesting how, when classes do appear, the following classes seem to suggest themselves: Command, Medic, Engineering, Scoundrel, Jedi, Noble. I probably would not use them all in one setting.