Perhaps before Universal opens up whole new worlds, they may first expand each existing world. Here are the arguments for each world and why they could be *first* to expand, as well as ideas for what attractions could come.
Wizarding World - Paris
We know Paris was supposed to get a second ride, but the idea got scrapped. I find it likely that Warner Bros will want to see a fast-tracked replacement to an attraction they were essentially promised. It's also very likely that Universal / Warner Bros may have an outline of an attraction (eg general concept, story, etc) that they could retool into something that works.
I think there are two major themes missing from The Wizarding World. First theme: flying broomsticks. This was the original rumored VR based attraction which got scrapped. I'm betting the fact that it was broomsticks had little to do with its cancellation. Could a ride use broomsticks but without VR? A ride system like Avatar Flight of Passage, Tron Lightcycle Run, or perhaps another unique ride system could work here.
The next missing theme from the Wizarding World is Dumbledore, who only appears briefly in Forbidden Journey. He's an integral character, and could be used to tie the two movie series (Potter and Fantastic Beasts) together. An attraction where Dumbledore teaches a sort of history class, taking students through critical moments in the Wizarding World timeline (as observers) could work very well. We could relive some favorite movements in a sort of 'best of' experience, without worrying about messing up Potter canon.
Dark Universe
The argument for expanding Dark Universe is simple: it's the smallest land. I think it's likely that this is the land Universal wasn't sure how guests would react. The reviews so far are overwhelming positive, with the major criticism being that the land is just too small.
Because Dark Universe is first party, this could give them a bit more freedom to fast-track an attraction, as opposed to potential red tape which may be involved working with WB and Nintendo.
The Creature from the Black Lagoon water ride ideas floating around (pun intended) sound like a good fit.
Super Nintendo World
SNW has had overall positive reviews, and it seems to be very fleshed out. So why would SNW be the first? Almost zero people are rating Mario-Kart or DK as the top attraction at the park. Typically the top three are: Monsters, Ministry, and Stardust (not necessarily in the order). From there we might see MK or DK as 4th or 5th, but even then they have tough competition with Hiccup's, Untrainable, and the Paris Show. So if you're the Nintendo leadership, how are you going to feel knowing your rides are not the best at Epic Universe? It's possible that Nintendo leadership gets jealous and pushes Universal to build another e-ticket Nintendo attraction to rival Monsters/Ministry/Stardust.
The three obvious IPs are Luigi's Mansion, Pokemon, and Zelda. Not sure what ride system they could use. But within this 'Nintendo is jealous and wants to make the best ride' timeline, the ride would need to be crazy cool... Cooler than the 'MIB but you're vacuuming up ghosts' idea that we've seen with Luigi's Mansion.
HTTYD
I think this land has the least amount of justification for why it should be first to expand. It's a very solid land, with a great show, family coaster, flat ride, water ride, play area, and character meet and greets.
There is one timeline where the upcoming live action movie makes crazy amounts of money at the box office, and Universal leadership decides to capitalize on the success by fast-tracking an attraction. This seems pretty unlikely though.
If anything is really missing from this land, I think it's a 'ride on a REAL dragon' experience similar to Avatar: Flight of Passage. Something to one up Disney with this concept is what I'd go with.
Celestial Park
If Universal leadership feels they need some quick added capacity / additional attraction, then Celestial Park may be the first 'land' to get an attraction. They could very easily toss up a lightly themed flat ride or coaster in several different spots connected to Celestial Park. Stormforce was the first ride was added to IOA, so there's precedence set for Universal adding a flat ride as the first new attraction.
Note that Epic Universe is the only UOR park to not have a spinner flat ride (Kang and Kudos at USF, One Fish Two Fish at IOA). An astronomy themed spinner, possibly with some heights rivaling Astro Orbiter, might be a good touch and would give some nice views of the park. And this would not at all ruin sightlines if placed in the expansion pad between the entrance and SNW.