In the sci-fan supplement that we are working on, we have narrowed the field for character classes to the following:
Tech: Short for Technician. Basically the everyman from the sci-fi setting. What I mean by this is that he is the "average person" who has found himself becoming an adventurer, and because technology is prevalent in the sci-fi setting, the average guy is a tech. In the same way, in my mind, that the fighter/fighting-man is the average guy in the fantasy setting--let's face it, all he has to do is pick up a sword and go adventuring. Nothing about the fighter/fighting-man requires special talents or abilities--just a desire to go out and swing a sword. His primary attribute will be Intelligence.
Psion: The person gifted with extreme mental abilities, the psion functions in many ways like a typical spellcasters does, except obviously his abilities do not come from an ability to channel arcane energies but from the ability to harness the powers of his mind. His primary attribute will be Intelligence--I think. An argument has been made that Wisdom might be appropriate.
Soldier: Trained in the use of military hardware, the soldier is the professional combatant from the modern (futuristic) world. He is adept at using hadnguns, lasers, artillery, etc. and gains a slight bonus when doing so. At the same time, he is physically tough and doesn't shy away from hand-to-hand combat.
While there are ideas for other classes, I think that for the sake of keeping the project short and (relatively) manageable, we'll stop there. Perhaps future products will include more.
As far as races are concerned, we will definitely include robots at a playable race. The inclusion of the robot is going to be one of the toughest aspects of the game to get right--both mechanically and stylistically. The initial conception of the robot is a creature that has a set number of hit points (which do NOT increase with level) and a static set of skills (because earning experience points and 'gaining levels') doesn't work for a mechanical being who is programmed.
Obviously, those two points greatly restrict what a robot character is and can do--especially in a game that is built on the level-up mechanic--but there are many game systems that do not rely on that mechanic which can be just as fun as D&D. The important question becomes how do we insert a non-leveling race into a game such that it still fits well in that type of game, still provides the player with enjoyment, and is not so overly different that it just doesn't feel right within that context. I think that this will be the greatest challenge.
One could argue that there isn't any reason that robots couldn't gain experience and level--but for some reason, that I can't put my finger on, I like that limitation on the robots. Perhaps, in my mind, the static (versus growing) robot just seems more pulpy to me.