2 roles that would be great to add is medic and detective. Now that there's 12-15 players, these roles are much more easier to add. For example; Game of 15, 2 impostors, 1 detective, 1 medic and 11 innocents. The detective can witness a killing, and kill an impostor instantly, however everyone will be noticed. However they could also report a body, and decide to inspect a player, or kill them. Killing the wrong player causes both the player and detective to die.
For the Medic if they find a body first they can choose either to revive or report, they can also med scan(Not actual task) where it can show that they're the medic.
Now that there's 2 roles to buff the crewmate side, the impostors also need a buff, Though i'm not sure myself, the best option is to provide other ways to kill players, for example traps, but limit them to prevent trapping, along with having other impostors susceptible to the raps, forcing them to work together.
Sorry about this long thread, just wanna help out with his game :)