6. The Terileptils
We know a little about this enemy from the Fifth Doctor story, The Visitation: the Terileptils come across very impatient and hostile, and they are the ones that broke the Doctor's sonic screwdriver for the rest of his era. We wouldn't see another sonic screwdriver for quite awhile, until the TVM featuring the 8th Doctor. The Fifth Doctor offered to take them home, but the Terileptils were not interested at all, instead choosing to try and kill everyone on earth instead, but there are new creative ideas writers could come up with if they re-introduced the Terileptils.
5. The Ogrons
The Ogrons appeared in the Third Doctor episode, Frontier in Space, and quickly disappeared afterward. They seemed to be more of a race of violence, but they came across rather dim-witted as well, but again this was an enemy that was only used once and then was put aside, only being heard of onscreen. Many people may not want to see the Ogrons back, simply because they weren't as big as a enemy as say, the Daleks or Cybermen, but it would be good to see them return, just to see where they came from and if and how they've changed since the Doctor saw them last.