Warning! SPOILERS ahead for The Flash season 6, episode 2.
The Flash dropped a reference to a major hero from DC Comics: Mister Miracle. That makes Mister Miracle the latest member of the Justice League confirmed to exist somewhere in the Arrowverse. This week's episode of Batwoman name-dropped Wonder Woman, one of the biggest characters from DC Comics still missing from The CW's shared universe of DC shows.
In The Flash season 6, episode 2, "Into the Void", Barry (Grant Gustin) decides to travel into the future to find out for himself what will happen in Crisis on Infinite Earths. Barry is injured in the process, and travels to Earth-3 where he gets help from that timeline's version of the Flash, Jay Garrick (John Wesley Shipp). Jay uses a device to help Barry find out what they're dealing with - essentially turning Barry into DC's Doctor Strange - and Barry comes to understand just how dangerous the Crisis is to the multiverse. The Crisis threatens to destroy all timelines, as evidenced by the Arrow season 8 premiere.
Jay Garrick has a map of the multiverse which plots the spread of the antimatter, and he's identified certain Earths based on the heroes that are there. One of the timelines in the multiverse is labeled "Thaddeus Brown". In DC Comics, Thaddeus Brown was a circus escape artist who wore a colorful costume and went by the stage name of "Mr. Miracle". Created by legendary comic book writer and artist Jack Kirby, he was a short-lived character who died in the first issue he was introduced (Mister Miracle #1 in 1971). His mantle was passed on to a member of the New Gods, Scott Free. Free made "Mr. Miracle" an important name in DC Comics. Over the years, Mister Miracle and his love interest Big Barda have developed ties to the Justice League. As part of the New Gods, Mister Miracle and Big Barda are also connected to one of DC's most powerful villains, Darkseid.
The Flash - like all of the Arrowverse shows - is continuing to pull other DC Comics characters into their universe, and based on what's to come in Crisis on Infinite Earths, the Arrowverse isn't going to stop expanding anytime soon, so it does seem possible that a later episode of The Flash or the crossover itself could explore this Thaddeus Brown reference in more detail. The character may even appear in person, perhaps as a setup for the franchise's next villain - Darkseid - should the producers choose to go that route.
Regardless, it seems unlikely that Scott Free's Mister Miracle will make an appearance in the Arrowverse at some point, especially since there's a strong chance he'll appear in an upcoming film set in the DCEU, New Gods. Either way, the mention of Thaddeus Brown is a fun reference to another important corner of the DC Universe that deserves some level of representation in the Arrowverse.