Hmmm...
I could deal with Androids as playable, but it would have to be done well, in a way that is beneficial and respectful to all players, not just Data fans. The androids would have to be a different type/mold than Data, since androids in popular service would be just getting started and thus would not be as advanced as Data. This would help bring balance since Data was so totally uber.
The other thing about androids is that they're not robots... all of their parts are integrated so tightly, its not like they could just upgrade an arm on stuff... they would need entire, intrusive, systems upgrades when they got them. Could be interesting. The bottom line is that they could have their own advantages, that is, certain inherent invulnerabilities, and higher physical stats, which could be balanced by a slower skill acquisition rate, or something else of that nature.
At the same time, to keep Androids from overpopulating the uni, there would need to be some kind of gateway. I would propose a different gateway for each of the 'special crewman' types that we've seen, and some new special crewman types, honestly. Holograms, Androids, Freed Borg, Cyborgs (Geordi), Betazoid, Joined Trill... each of these guys have inherent strengths that are not available to all starfleet, and are 'special' in ways that it would make sense for them not to be available when you first make your first character, or just buy an expansion pack.
For Androids, I often suggest that challenge be to 'max out' a character, since, honestly, that's what an android is, someone with maxed out physical stats, high skills, and not really a whole lot of advancement. Besides, if someone trades in their Picard for a Data... I'm not really going to feel like that's 'unfair.'
Other gateways for other specials could include: Random chance at login, collecting sets of trinkets from around the galaxy, a high level epic raid quest, exploring all the quests of the target race (complete all the Cardassia missions, you can play a Cardassian member of the federation), maxing out some other unknown stats (ie Social Skills and contacts to unlock a Betazoid) and etc...