Well, it depends on what Nintendo wants. For what it's worth, USF will probably have attendance of around 12-13 million by the time SNW is ready to open there. The new resort's park will likely open to around 6-7 million attendance in its first year and then grow pretty rapidly, but I'd be surprised if the 3rd dry park had over 10 million attendance before 2026. These things take time, and don't forget that the first resort will have around 10,000 hotel rooms while the second resort will probably have only around a half that by 2026-2027.
As for the second resort, I think most would be surprised if it opened before 2023; Spring 2023 is around 5.5 years away, which is a fair timeline given all of the land preparation that needs to be done on the main lots (a lot of the missile testing toxicity hasn't been completely cleaned out and there's a lot of changes that probably need to be made to where the bodies of water are).
But who knows at this point, if KidZone doesn't close before 2019, then you're probably right that they'd have agreed to wait for the 3rd dry park, but again, I think it's more likely that just agreed to hold one or two IPs for that park.
Not to mention, when it comes to UC right now; it is rather liquid. Especially since D23, we have had rumors of them wanting to put MORE into the development of the northern resort, with projects seeming to be more on the verge of getting greenlit (and some already greenlit).
It doesn't phase me that Super Nintendo World is being looked at for getting extra for the USF front, as it makes sense as a way to keep people in Orlando, even after the 50th Anniversary of Walt Disney World. What does phase me, is that Pokemon seems to be the one right now that will be the +1 in all this.