Honestly? There are way too many question marks to say for sure. Just offhand, a few of the variables in my mind would be:
-Is there a vaccine? If so, when? Is it early enough for people to book summer trips in 2021, or not until late spring so people are traveling further out?
-What does the economy look like? Are people still out of work? If so, a lot of people might not be able to afford a vacation.
-What are schools doing? Did they start back early in fall 2020, like some are talking about? Are any of them ending early as a result? (I think this is the least likely scenario, but some schools out here are talking about going to a shifted or split schedule or even moving to tracks so 1/3 of students would be out of school at all times).
-How long has everything been reopen? Think about all the people rebooking 2020 trips. Airlines are generally giving people a year to rebook, Disney is giving incentives for certain times but allowing full cancellation and rescheduling later, etc. Right now, most people who had spring 2020 trips planned seem to be rescheduling for summer or fall 2020. If they don't reopen, or only partially reopen, or people are still afraid to fly, etc. for fall 2020, then the people who had spring OR summer OR fall trips planned in 2020 would be rescheduling for 2021. On top of people who had already planned to go in 2021 for the anniversary.
-What does air travel look like? Still dirt cheap? More expensive because they can't use middle seats? Are people still nervous about flying because for some reason half the country can't work out how to put a mask over both their mouth
and nose at the same time? Are there more flights? Fewer?
-What does international travel look like? (Less likely to impact early May than other months, either for spring break or summer vacation, but still. If I would normally go to Europe but I can't because of travel bans, I may go to Disney instead. And if international travelers can't come here, that would decrease crowds).
-What is Disney doing financially? Did they increase costs to make up for the loss? Or did they, like after 2001 and 2008, start adding promotions on promotions to try to entice people back? Can I get free tickets if I sew masks for the local nursing home? Does everyone get
free dining now? Or are they blocking out half their hotel rooms to prevent the parks from reaching capacity?
Unfortunately, answering your question is kind of like answering "How long is a piece of string?" There are just too many unknowns to have a reasonable answer at this point. If I were going to guess, off the top of my head, based on what I know today, I would say less crowded than usual with mostly die-hard fans, making lines shorter but ADRs and FP+ harder to get because it'll be people who know how to plan stuff well in advance. But I'm guessing than 5 people could come up with 15 answers and all of them would be equally likely to be correct.