VGC reports that the mysterious door is located and 'hidden' towards the north of the park, featuring "a very DK-esque design" as seen in the image below. Neither Nintendo nor Universal has acknowledged this door in any official capacity so far, but there's plenty of reason to believe that new Donkey Kong elements are already being planned for the park.
Towards the end of last year, dataminers uncovered Donkey Kong stickers in the park's mobile app, suggesting that there will be activities to see and do featuring Nintendo's beloved Kong family of characters.
And rumours of DK's involvement with the park have actually been present since way back in 2019, when leaked concept models revealed a Super Mario area and Donkey Kong area. The Super Mario concept work is almost identical to the real deal seen at the park today, so it's not exactly a stretch to assume that the DK model was also legit.
Even if COVID wasn't ruining our travel plans, perhaps holding off on a visit is the best course of action? Two areas in one sounds pretty great to us.