I agree with having the Edosians (http://en.memory-alpha.org/wiki/Edosian). Might be hard to make Boffs or Captains with the extra arm & leg animation, but I'd buy a Boff in the C-Store. But at least having them as Doffs should be easy as long as they're part of the license.
The enemy species I'd like to see are the Kelvans. (http://en.memory-alpha.org/wiki/Kelvan) We saw them disguised as humans in TOS' "By Any Other Name," but the description of their natural form sounded like mini-Lovecraftian monstrosities. (Mini for Lovecraftian monstrosities, that its.) Worf claimed to have fought one, so they should be still around. Cryptic could flesh out a non-humanoid enemy that was beyond TOS' or even TNG's special effects abilities.