This is all my opinion. No politics behind this - just looking at the state of things and the numbers. Unfortunately I am still on the pessimistic side...
I would take cues from how states are handling sporting events and concerts, because amusement parks and HHN is not far removed from that at all.
I foresee some sporting events taking place in the summer - but being played in empty stadiums. Concerts postponed indefinitely.
I don't think parks will open in June, July, and probably not August at the present state of things.
HHN construction, and construction in the parks will continue. There will be completed houses come September. If the parks aren't open, then HHN will not be open. But I think they MAY find a way to open the parks to some degree to limited guests late summer, but there will be flare ups which may result in periodic closures. So I think it's possible HHN will take place. One thing is for certain - if it does not, they won't be tearing down the houses. I can easily see HHN take place during early/mid 2021 if it had to be postponed. It would be a moneymaker even if it was in April. I could even see HHN30 shut down mid-run and Universal simply keeping the houses in place and "resuming" the event during September 2021 in the exact same form. The parks are hurting terribly, and if it meant running the same houses two years in a row, I think many would accept that. If you think about it, Universal is probably in worse shape for Halloween as opposed to other events - because they tear down their houses every year. Howl O Scream, Knotts, Six Flags, and many local haunts just turn the lights off and keep the houses intact all year. They are much better prepared for this, but if they were open this year they, too, would need to make many changes - widening areas, removing SIF, adding ventilation, etc.
There are a few wildcards out there that will be played before September:
Some states are going to decide to open up a bit on May 1 or even sooner. This will undoubtedly result in some spiking of the number of new cases. How much remains to be seen. We should see spiking within two weeks time due to the recent protests. Really need to watch those numbers. The question will be how the states respond. Some will panic and snap restrictions back immediately. Others will watch cases vs hospital capacity. If the spikes are too strong, then any question of theme parks opening this year will be met with a resounding "Oh HELL no!"
Antibody testing is also a wildcard. We saw the one Santa Clara study of antibody testing imply that the number of actual cases is up to 80x higher than reported. Which means there are far more people who had the virus but were asymptomatic or it was that "really bad flu" people had in January. But we also don't know the level of immunity these people have (if any) or how long they will have immunity. But if further studies show a huge number of people who have been infected at some time, there will be massive pressure on states to open. Without widespread testing and the knowledge that antibodies provide meaningful immunity to coronavirus, I don't see parks or any crowded events opening by September.
The amount of Covid19 testing that has taken place is ridiculously small. 4 million tests out of 328 million people. And most of that testing has been done only on those with symptoms. And lab analysis is still taking multiple days (meaning it is impossible to test the nation in the next couple of months). Once widespread testing occurs, I believe we will see massive spikes, but we will also see that there is a much larger number of people who are infected but asymptomatic or not sick enough for hospitalization. The later this happens, the lower the odds of HHN happening are, because the response of the country will most likely be to overreact and clamp down hard.
Required PPE: Currently, many states are requiring masks be worn. Even if those restrictions are removed for those who have an "immunity passport", I don't see that being relaxed for mass gatherings - assuming mass gatherings are even permitted in September. This may work for theme parks, but how about HHN? If patrons are required to wear masks, employees would be required to wear masks. "Sure, that sounds fine. HHN heavily involves masks anyway." But this would include show performers as well as scareactors. We aren't just talking about Michael Meyers masks. We are talking about a mask under a mask. And will a cloth mask be approved? Because N95 supplies will be going to health facilities and not amusement parks. Now think about the costumes, and all the people who need to handle those every day. Laundering them. Repairing them. Prepping them for the actors. In short, I think mass gatherings will be the last thing that gets relaxed, theme parks are a unique subset of mass gatherings, and this successfully happening by October is unlikely.
Here are my foreseeable scenarios for HHN:
Most Likely: No parks open this year - no concerts/sporting events/mass gatherings. HHN 30 takes place ASAP, even if early/mid 2021. No matter what year it is, it will always be HHN 30. I firmly believe Universal could have HHN30 and HHN31 take place in 2021 if needed, changing only houses that need to be taken down for summer and IPs that cannot be extended. 2021 could be the "Year of Belated Holidays" with all holiday events happening at odd times. Why not?
Possible but unlikely: HHN takes place on schedule, but attendees must show proof of antibody results or no infection (a nationally required "immunity passport"). Possible that only Florida residents will be allowed entry and ID is required (because every state will have a different degree of safety). High likelihood of event shut down mid way due to new outbreaks/virus second phase/reports of new infections who recently visited ANY haunt or theme park.
Very slim: HHN takes place on schedule. Temperature screening only. PPE required. Limited number of attendees. Virtual queues.