An interesting study I found:
A new study of the more than 300 outbreak clusters of COVID-19 in China reveals that the majority of the outbreaks were fueled by indoor transmission of the disease, while outdoor transmi…
thehill.com
I wonder if it would be possible to only open outdoor offerings. The only issue with that, is that there are very few places/attractions that are completely outdoors. MK only has Jungle (and the temple could negate that possibility), Carpets, Liberty Belle, Dumbo, Barnstormer, Speedway, Teacups, and Astro (which the elevator could negate). Admittedly DAK has the best outdoor to total rides ratio thanks to Safari, Kali, Primeval, Tricera, and Everest (which the brief indoor portions could negate). Meanwhile DHS would only have 2/3 of Toy Story Land to work with and Epcot would have nothing. With the possible exception of DAK, I'd unfortunately doubt that "outdoor only" would feasibly be enough to open the park.