TV Head Superman makes an appearance in Thunderball. This is a subtle nod that the James Bond Extended Universe overlaps with Bizarro World.
A Christmas Story takes place in the DCEU. In this universe, Earth 9028128357432604, Mother Parker (Melinda Dillon) is Harley Quinn.
Jason Mamoa plays the titular role in the movie Aquaman. This is a subtle nod to the racism that still exists in Hollywood. I mean, why else would they cast a man of Native Hawaiian ancestry as the worst superhero in the DCEU?