A list of all Universal Orlando rumors, as of late March 2018. This list is pure speculation, nothing is confirmed. Rumors are divided into categories. This list only applies to the current parks, all Uni. South speculation is to go into another thread.
Universal Studios
Super Nintendo World- Rumored to replace Kidzone. The land would include attractions based on Mario Kart, Donkey Kong, Yoshi, and Bowser Jr. The attractions would feature multiple animatronics and set pieces. Other restaurants, shops, and play areas would be spread throughout the land.
Ministry of Magic- Rumored to replace Fear Factor Live. Nothing else is currently known about this attraction other than its rumored opening date, which would be sometime in 2020.
SLOP/ Villain-Con- Rumored to replace Shrek 4D. The attraction could feature a trackless ride system. SLOP would be a story-based ride, while Villain-Con is speculated to be more of a museum kind of experience, with the ride vehicles showcasing weapons found at the event.
Pokemon- Rumored to replace Kidzone. Nothing else is known at this time.
Hello Kitty- Rumored to replace Kidzone. Nothing else is known at this time.
Splatoon- Rumored to replace Kidzone. Nothing else is known at this time.
Bourne Show- Rumored to replace Terminator. Set to open in 2019.
Islands Of Adventure
Harry Potter Roller Coaster- Replaces Dragon Challenge, and is set to open in 2019. Said to be one of the most technologically advanced rides in Universal. No story details are known at this time, although it is speculated to revolve around Forbidden Forest or the Tri-Wizard tournament.
Lost Continent Replacement- Rumored to be the kingdom of Hyrule (of Zelda fame). Nothing else is known at this time.
Jurassic Park Expansion- Jurassic Park is rumored to get a Jurassic World overlay and a new ride. JPRA could get an upgrade of some sort, possibly featuring new animatronics. Nothing is currently known about the new ride.
Iron Legion- Said to be delayed/cancelled. Sorry Marvel fans.
Other Miscellaneous Rumors- A Toon Lagoon revamp/retheme and a Seuss expansion are also rumored, but not much is known about them.
Volcano Bay
Expansion- Rumored to include a new water coaster. Nothing else is known at this time.
If I didn't get all the rumors, please post them below.
I would classify the expansion to Volcano Bay and Nintendo confirmed in their own regards, but for Nintendo, it's more of a complicated beast than anything.
Compiling a list of my own, and this is pretty much what I have in terms of reading.
USF
The Future of Shrek 4D - This has been a rumor popping up more, with two possibilities being Secret Life of Pets, or a brand new Minions attraction themed to the Villain-Con location seen in the 2015 movie. Both sound like they'd be using LPS technology of some kind.
Ministry of Magic - It has been rumored (and somewhat leaked if you count a certain interview around the time of Diagon I think) that MoM would come to London. The only location that would make sense is Fear Factor Live, and presumably would extend the London Facades to border the Men in Black - Alien Attack extended queue.
KidZone - ....Well, this is an odd one to talk about. It seems as of now there lies multiple angles. One could be Super Nintendo World, with Donkey Kong; Yoshi; and Mario being major attractions. But then there lies the possible idea of a Japanese oriented land instead, with Splatoon, Sanrio, or Pokemon used for the land. As of now, I don't think we can put any form of stock to what is coming, with maybe the bare exception being SNW due to previous permits
Bourne/T2 Replacement - The Terminator 2 replacement has seemed stuck on two angles, with Bourne being the most likely out of that and Bond. They are apparently expanding the armory of the show, and should have some elements of similarity to T2 while at the same time being something brand new.
IOA
Hogsmeade Dragons Challenge Replacement - The new Dragons Challenge replacement sounds like it will be an Intamin Sit-Down coaster of some kind, with Life Formations and other groups for an Indoor/Outdoor, Highly immersive dark coaster. Originally, it was mentioned that Hagrid may have a role, but that it also seems plausible that it may fuse elements from the iconic Forbidden Forest and the story of The Goblet of Fire, and the Tri-Wizard Tournament into the attractions story-line.
Jurassic Park - Been rumored a few times in the past of a River Adventure revamp, but Hollywood seems to be the only one in line for it as of right now, unless if things have changed for it to be a project happening shortly after the DC Replacement
The Future of Lost Continent - Seems like this has been possibly earmarked for Nintendo's The Legend of Zelda. Previously, rumors circulated on Berk, Christmas Wonderland, and a few other things.
Misc - Something Marvel has been on and off the table, something The Grinch oriented has been on and off the table, and Toon Lagoon was slightly rumored previously to have Pets before it moved to Shrek. Honestly, nothing much can be said here.
Hotels
Dry N' Tame - Two Hotels will be building south of UOR, with the first to presumably open sometime in late 2019.
Misc
UOSouth - They are undergoing the process of a lot of paperwork to possibly get even more land for their project down South. Some rumors have pointed Lord of The Rings, DWA, Nintendo, and other things being headliners, with hotels consisting in the past being The Golf Channel, Cirque, and a few others into the mix.
Volcano Bay - Expansion is planned, as work may begin sooner than later for presumably a new slide complex.
And to bipedal on the holding off, Orlando isn't the only one having their cards all mostly set up, so I would wonder if there is something more to the scenes on that...