Too early to predict 2019. As far as 2018 goes, Aamir or Salman may end up grabbing Highest Grosser of the Year. With SRK, its very hard to predict the outcome because nobody liked what saw with the title of the film being announced followed by a sneek peek of the movie. He could suprise everyone but I don't like his chances at all considering people are losing hope in him with his choice of movies.
No questions about Akki as he's currently on a roll with back to back successful movies and he may end up repeating the same scenerio like 2016 & 2017. If Hrithik's film gets a very good response which I'm sure it will then there's 99% chance of him giving a Hit.
Now last but not least, Ajay Devgan. I'm a huge fan of him and I'm quite honest about his BO situation. It's more worrying for Ajay and less worries about Khans, Hrithik & Akki if we look at it from a BO standard. He hasn't given a Hit without Rohit Shetty but I think Total Dhamaal has the potential if audiences do accept the film. I don't know if Raid is really a niche movie but I would expect a repeat like Drishyam e.g. Average at the BO. His upcoming Rom Com film with Tabu & Rakul Preet looks bad on paper and its Akiv Ali's directional debut so nobody knows the BO outcome. It may Flop but if it turns out to be a suprise Hit then it will be a relief because all this talk about Ajay not being capable of giving a Hit without Rohit Shetty can be put to rest.