As there's no news of an actual new Smiley novel, it's all speculation. However, in the past, le Carre's Smiley novels were known for their nuanced portrayals of spies. If a new one were to come out, it might continue to show the inner workings of intelligence agencies and the personal struggles of the characters like Smiley. His writing often delved into the psychological aspects of the spies, so perhaps more of that in a new novel.