If you were an innocent or detective the previous round, you get two entries in a player list that it chooses from at round start to receive the traitor role. Without this extra entry you already have a 25% change to get traitor. I do not think that making a systematic system to select roles would be beneficial to TTT. Part of the experience is going through maps where you get traitor almost every round, or maybe the opposite where you get it one round. All of that is what makes up the gamemode, and if you know that you aren't going to get traitor next round after being a traitor the previous round, you won't have any motivation to play the next round. I agree that a ton of randomness when it comes to TTT can be bad, but right now where it stands with role selection, I don't think changing it would be beneficial.