8. The Mara
The Mara has always been an interesting enemy - sure the Fifth Doctor may have destroyed the snake in Snakedance, but who knows what the writers could actually do with the enemy this time around. It was fantastic to see Tegan's mind taken over twice by the Mara, and to see how evil and twisted the creature really is, and the prospect of witnessing the experience with maybe the 11th Doctor himself, or a newer companion would be spectacular to see. The Mara is one of those enemies that doesn't need much costume wise, but definitely could use a little bit of a touch up on the special effects area.
7. The Sea Devils
The Sea Devils first appeared in the Third Doctor era, in the episode The Sea Devils, before later appearing in the Fifth Doctor episode Warriors of the Deep, and like the Silurians, they never returned to the screen. The Silurians have however returned in the new series, and much to the dismay of many Classic Doctor Who fans because they look nothing like the old ones from the Pertwee/Davison eras. Should the Sea Devils return, they could be well left alone as fans would not be too happy to see another reptilian species looking more human than reptile.