7. The Rutan
The Rutan are a rival race to the Sontarans, and have been engaged in a war with them for over a thousand years. While they may not be as iconic as the Sontarans, it's quite strange how these aliens have never been re-used by the writers.
Perhaps it is because they aren't exactly your standard alien. Rather than having a humanoid or beast like form, they instead appear as floating green blobs, a bit like Jellyfish. This might not seem like the greatest monster to bring back, but it would certainly been an interesting change of pace to see The Doctor face off against them.
They could even be utilised in a Sontaran story. Perhaps the war between the two alien races ends up spilling over into our own solar system. However, we have seen that before so perhaps the Doctor could get caught up in the final days of the thousand year long war. With the Sontarans appearing several times during the modern era of the show, it seems a missed opportunity to not bring the Rutan back too.