A month-by-month look:
January: A slow month as usual. Glass will be probably be the biggest movie this month
February: Biggest movie this month might be any of the two animated movies released; How to Train Your Dragon:The Hidden World or The LEGO Movie 2. A wild card is obviously Alita: Battle Angel. It doesn't have any real competiton. But I think it depends on the reception the movie gets and if it can find success in markets like China. Speaking of China, its not impossible that the biggest movie this month turns out to be a Chinese movie, considering they usually release some of their biggest movies in February, in time for the Chinese New Year.
March: If nothing really surprising happens the Disney-onslaught begins this month with Captain Marvel. Dumbo will probably do well also. Although with three of these live-action remakes released very close to each other we might see at least one them maybe underperform somewhat, if not exactly be a flop.
April: Avengers: Endgame. Nuff said. I wouldn't be suprised if it brings a new opening weekend record, considering most superhero-movies beat expectations in 2018. And I think it can beat Avatar domestically.
May: With Avengers gone from this month, May might be a little more open race. But the odds are proably not so high on Disney taking another victory with Aladdin. I don't see anything else having a realistic chance reaching a billion worldwide. But, as I said before, having three of these live action remakes so close to each other might affect one or more of them. And some other movies might overperform.
June: Toy Story 4
July: The Lion King. Unless people are really tired of these live action remakes at this point and there have been a lot of negative feedback for the previous two. But what else is there? Spider-Man I suppose. That feels like a tricky one to predict. On one hand it's the seventh Spider-Man movie in total. On the other hand it's just the second in MCU, and it's the first post-Endgame MCU-movie.
August: Hobbs & Shaw. Sure, spinoffs can go either way, which 2018 showed with the disappointment of Solo and the somewhat surprising success of Venom. But as one of the summer's last big releases it might a good chance to thrive.
September: It: Chapter 2
October: Joker will try to copy Venom's success with an early October-release and why not?
November: Disney likely strikes again with Frozen 2.
December: A year ago I'd say this would be an easy victory for Star Wars. Still think it will win, but especially on international markets the race could be tighter than expected. A movie like the Jumanji-sequel is probably bound to be a much bigger hit in China for example