mass-effect-3
In my playthrough:
Thane was critically wounded by the Cerberus assassin on the Citadel. He died later in the Huerta Memorial Hospital.
Shepard and squad stood around being useless while Thane was fighting with the assassin. Is there any way I could have prevented this outcome?
A human doctor will be looking for the medi-gel formula you have in your possession following the completion of your mission on Palaven. You can find her in lobby of the Huerta Memorial Hospital on the Citadel, to your left when you walk in the door:
Outside of having completed the Palaven mission, there's nothing in particular that you need to do to trigger this mission beyond talking to her once in possession of the forumla. As Raven Dreamer notes in the comments, the doctor does have a side-conversation that would clue you into her need for the formula to begin with which you'll overheard as you approach her.
Your class is set once you pick one on a new import/creation for ME3. Unfortunately, your class is retained when you start a new game+ so you cannot change it. Bonus powers, however, are retained by profile so you can pick a different bonus power.
Best Answer
There's only one way where Thane survives the attack:
However, I wouldn't mourn Thane's death. If you speak to Thane prior to the attack, you'll learn that he's near the end of his life regardless of what happens. He says that one doctor gave him 3 months to live 9 months ago. He also talks about his constant pain and feelings of suffocation. For him, this is likely an honorable and worthy end.