Allow me to throw in my two cents.
Stranger Things 3
It's only obvious. We don't know the specifics of the Netflix/Universal contract (well, at least I don't) but I'm assuming it was for at least three years. I don't think the negative reception of this year's maze will hurt the property's chance of returning. Plus, Murdy hinted on Twitter on a few occasions that he was saving aspects of season 3 for future use. Hell, with season 4 rumored to be released in 2021, we have a lock for that year too.
Candyman
Whether the maze would be inspired by the original, Jordan Peele's remake (due this coming June), or a mixture of the two is beyond me but the property is ripe for the HHN picking. The remake is a Universal release, so that checks off the synergy box. Peele was also eager to bring Us to the event and seemed really happy with how it turned out. I don't see why he wouldn't jump at the chance to do another maze. HHN has been in dire need of a classic 90's horror property.
Beetlejuice
This one is admittedly a little blue sky, but I have reason to believe it's possible. Warner Bros' haunt is donezo for the time being, so I could see them easing up on their properties to make some sweet licensing money. After seeing how Ghostbusters was handled, I know a Beetlejuice maze could be amazing. It has way more scare potential and they could have so much fun with Tim Burton's style. BJ is also no stranger to the park, so there's that (I know the rights are different, but please indulge me for a second).
Zombieland
This one's pretty simple. I'm not too sure what Uni's relationship with Sony is like after Ghostbusters, but I can totally see them giving the property the good ol' Trick 'r Treatment. I haven't seen Double Tap, but the first movie has more than enough elements to flesh out its own maze.
Universal Monsters
I'm almost certain that Murdy plans to keep the monsters a staple of the event for years to come. I'm just not 100% sure how he plans to integrate them this upcoming year. Now, there are two potential scenarios here. Either the maze focuses on another monster as seen in 2018's maze or it's based on Blumhouse's Invisible Man movie (due early next year). The latter is much less likely given Murdy's rocky relationship with Jason Blum, but it's certainly still possible. The former, to me, would most likely be Dracula. More Frankenstein (as much as I love him) would be overkill at this point and Drac really got the shaft in 2018's maze so it makes plenty of sense to me.
John Carpenter's The Thing
Okay, this is really just me hoping for the best. It's one of Murdy's white whales and it's been speculated for years, so I'm hoping it finally comes to fruition. It's also not a crazy idea since the creative team now has the opportunity to properly allocate the budget to nine different mazes. I would gladly take another neon recycled prop extravaganza ala Pandora's Box if it meant having enough money for all the puppets and creature FX The Thing would need.
Mexican Original
Holidayz and Pandora's Box were huge hits so more originals seems like a no-brainer. Murdy's been sitting on the Mexican original for a little while and now seems like the perfect time to bring it to life.
Music Maze
This is just me spitballing. We haven't had a purely music-based maze in a little while so I could see one happening now. Murdy has said time and time again that he'd like to cap off his Alice Cooper trilogy and Metallica's Kirk Hammett is a huge horror buff so those two in particular really stand out to me as possible candidates.