In his first scene, Novu comments that his suit "looks ridiculous." When he approached Laurel in "Present Tense (2019)" and requested that she betray Oliver in exchange for the restoration of Earth-2, she deduces who he is because "no human would actually wear something so ridiculous."
In the comics, Planet Maltus was the original home planet of the Guardians of the Universe and one of the most over-populated planets in the universe.
"I'll light the spark, you fan the flame" is one of the many meta-touches this finale had regarding Oliver's role, and especially Arrow's role, in the shaping of the Arrowverse. Arrow did indeed light the spark, and all of these other shows have fanned the flame.
Ezra Miller: His cameo was added after filming for the crossover was wrapped and the cast didn't know about it.