10. Bringing Black Lightning and Supergirl to Earth-Prime too late
I cannot tell you how excited us Arrowverse fans were to see Black Lightning show up on Earth-1 in the Crisis On Infinite Earths crossover. But that would only be the beginning, as the crossover itself concluded with the show - which was initially envisioned as a non-Arrowverse show - joining its Arrowverse counterparts on the newly-created Earth-Prime. Similarly, the characters and setting of Supergirl - which had crossed over with The Flash multiple times before - was also now set on Earth-Prime.
The prospect of more interactions between all of these shows was thrilling. However, the Arrowverse was never able to capitalize on it. Between the pandemic preventing any immediate crossovers to the subsequent endings or cancellations of shows within the shared universe, there wasn't enough time to realize the vision of what Earth-Prime could accomplish.
We had a Justice League ready for another major crossover, but it just wasn't to be. These shows missed their opportunity to make this happen much earlier in their runs with such momentum, and as awesome as the cameos from Black Lightning and Supergirl's Alex Danvers in The Flash's Armageddon special were, it might have been best to keep the shows on different earths instead of doing it this long into the game.