Who says the Geth isn't a woman already?

For the rest, well, Geth...you know, they're robots. You could add a certain other robot into the game, though, that there's already a model for....
I'm sure its all programming issues. They could add a good other 10 models and levels etc. But the incentive in $$$ has to be there too. Probably they're still concerned with bugs...like flying VanGuards...and then, you know, have to add other content too.
Having said that, more character models is always better. They could, in fact, probably spend their time releasing a multi-player only game if they can smooth out these bugs. Something with a ton more options. Here's to hoping!