In "Crisis on Infinite Earths," fans were given an unexpected treat when Tom Ellis appeared as Lucifer Morningstar, revealing that the hit series Lucifer is part of the Arrowverse. With this cameo, viewers learned a number of important details, including that Lucifer is well aware of the multiverse, and that he and John Constantine have some history.
So, with Ellis officially returning for another season of Lucifer on Netflix, it's time for the series to return the favor and cross over with the Arrowverse. And while there are any number of plot points that could be used, the one with the biggest potential would include Matt Ryan's Constantine.
Naturally, this crossover would require going back to the past of the series before the events of the first episode, but it wouldn't be the first time that the show has spent an entire episode as a flashback. The obvious choice for such an episode would be to show just what Constantine did that caused Lucifer to be in John's debt. The only hint fans have so far is that it had something to do with Maze, but this could be expanded on significantly, especially if it were to explore just how Constantine ended up on Lucifer's Earth in the first place.
Another possibility for the tie-in would be to reference just how much "Crisis" affected Heaven or Hell. Due to circumstantial evidence, it can reasonably be determined that the event took place prior to the actual show itself, so there's not much that would have noticeably changed in the world. That said, Legends of Tomorrow has already shown that the supernatural beings of the Arrowverse were affected by
"Crisis," some for the better and some for the worse.
This could certainly tie into the plot of Season 5, with the ramifications of the "Crisis" playing a role in what ends up happening over the season, and possibly even previous ones. It would certainly provide a retroactive explanation for Lucifer's lack of concern over what would happen if his Mother and Father ended up fighting in Season 2 and what that would mean for the universe. After all, he already lived through the destruction of the multiverse at the hands of the Anti-Monitor and it's subsequent rebirth thanks to the Spectre, so even the collateral damage of God and Goddess fighting would likely be minuscule in comparison.
Ultimately though, the easiest and arguably cheapest way to have a crossover with the Arrowverse would be for one of the characters to simply mention the "Crisis" in passing, or in the case of Maze or Lucifer, mention Constantine by name. It certainly would be in character for Lucifer and Maze to treat the whole thing with total irreverence, especially since Lucifer's cameo during the crossover had him doing just that.
It certainly wouldn't be the most exciting way to bring the Arrowverse back into Lucifer or vice versa, but in terms of behind the scenes complications, it would definitely be the most efficient way. Regardless, with either a cameo by John Constantine, another appearance in the next Arrowverse crossover, or something else altogether, fans eagerly await whatever Lucifer and the Arrowverse have in store.