Pacific Rim: Uprising makes a lot of changes to the first movie by focusing on a lot of brand new characters. The first movie saw Charlie Hunnam in the lead but the sequel will see an entirely new character played by John Boyega take center stage. A few characters from the original Pacific Rim will be back for the sequel but apparently, there were originally plans for a lot more. Writer and director Steven S. DeKnight recently revealed that there were plans to bring back Ron Perlman's Hannibal Chau, as well as a handful of other characters from the original film. According to DeKnight...
I had actually written a cameo for Ron Pearlman but the scheduling didn't work out, but you know, there are so many great characters in the first movie. There's Max Martini as Herc Hansen. At one point he had a really nice role and I had to change the story. At one point he was the Marshall and that got changed and of course, I'd love to bring back Tendo (Choi played by Cliff Collins Jr.) for the movie, but ultimately I was trying to take all of these characters and give them a little something in the movie. Then the movie got so big I just had to tear it back.
The Pacific Rim sequel was originally going to look a lot more like the original movie than it has ended up looking. Guillermo del Toro had been trying to make the sequel happen himself for a long time but when the production finally moved forward, it was without him as director. Charlie Hunnam was interested in returning as the lead, but a scheduling conflict prevented that from happening. It seems that continuity with the first film was going to be found by bringing in other characters instead, but in many ways that didn't work out.
Steven DeKinight tells ScreenRant that Ron Perlman, like Charlie Hunnam, just couldn't make the schedule work. Max Martini, who played "Herc" Hansen in Pacific Rim was originally going to have an important role, as the new Marshall, the position held by Idris Elba's Stacker Pentecost in the first movie, however, his role ended up being eliminated through script rewrites. Cliffton Collins Jr. was also going to come back as Tendo Choi, but it sounds like there wasn't really enough for him to do, making it not worth it to bring him back.
Pacific Rim: Uprising won't be entirely without returning characters. Charlie Day and Burn Gorham will both be back as the pair of squabbling scientists and Rinko Kikuchi will be back as Mako Mori, who stole the show in the original movie. Pacific Rim: Uprising arrives in theaters March 23.