I like Morbus currently. It's similar to TTT. The "Traitor" is a human shapeshifting alien and needs to kill the survivors. Killed survivors spawn as aliens too (either same as the Traitor or as a weaker version, depends on how the survivor got killed)
In my opinion it's more balanced than TTT (no instant kills) and it feels like playing a game based on the movie "The Thing" . The only bad thing are the maps, which could get repetitive (all similar looks). TTT has much more variety in terms of maps.