2. The Sontarans
This is very much like the Silurian situation. Strax is a funny character that really makes an episode enjoyable to watch. But after seeing a Sontaran in a top hat, it's hard to see them as scary again. Much like the Silurians, I feel that the Sontarans should not return until Strax is no longer in Doctor Who. If they were to return, the best route to go down would be to finally see their battle with the Rutans. It would give Whovians a chance to see the angry and trigger-happy Sontarans we all know and love again. But I would much rather see the Sontarans return than I would...