I did a search first, but hopefully I'm not pointing out well-known cut content.
First of all are soundsets, Dodonna and Vandar both have soundsets. Perhaps you were meant to be able to fight them directly if you went darkside? Also, it seems that BioWare originally intended for the player to have a different soundset depending on if you picked one of the "black" heads. Personally, I can see why they removed that, but there's still a few lines in the game files, in the streamsounds folder, all of the file names begin with n_dth_revfb and n_dth_revmb. Additionally, there are a whole bunch of alternate versions of lines for the PC's soundset. Using the female PC as an example, when selecting her from your party she normally uses the p_plyerfw_slct1, p_plyerfw_slct4, and p_plyerfw_slct7 sounds, but BioWare included p_plyerfw_slct2, p_plyerfw_slct3, p_plyerfw_slct5, p_plyerfw_slct6, and p_plyerfw_slct8 as well.
I'm not sure if they were unused or not, but there are two generic commoner heads which I don't remember appearing in the game. N_comm_a_F and N_comm_a_M. I can't find the head models for either of them, but they're both based off of PC heads.
Lastly, I found an incomplete, untextured model for an alien, l_alien01. He looks like Admiral Ackbar's species to me, but he might be something different.
I don't think it's possible to put him in the game proper, but it's interesting, nonetheless.
