Thanos is a warlord from Titan who believed he was restoring balance to the universe by eliminating half of the population. He believed the universe was overpopulated, and that if he did not sacrifice half of those alive, then the entire population would suffer due to a lack of resources. He can be seen as sort of the 'final boss' of the Marvel Cinematic Universe. With the new DC Extended Universe films being released—such as Robert Pattinson's The Batman—the Joker should be to the DCEU what Thanos was for the MCU.
Due to overpopulation, Thanos experienced the destruction of his own home planet which is what caused him to seek stability in the universe. Though this sounds like a responsible and worthy desire, Thanos's plan to exterminate half of all life is on par with some of the irrational thinking that the Joker displays. As a child, Thanos had a strange fascination with death that disturbed people around him. His behavior is similar to the Joker who is also fascinated by death as he kills innocent people for his own amusement.
Unlike the Joker, Thanos used to think all life was precious and wouldn't even think of harming anyone. An event from his childhood changed that. Thanos and his friends went into a dangerous cave to explore but got trapped inside for several days. There were creatures within the cave that Thanos wouldn't kill even though he was starving and needed to eat. He was separated from his friends and when he found them, they were all dead—killed by the same creatures he decided to spare. Because of this, Thanos grew up with the mentality that death was necessary for life to continue.
On the other hand, the Joker seems to be insane and without reasoning behind his crimes other than to have a laugh. But the recent Joker filmed showed a side to the character movie audiences hadn't seen before. Untreated mental health, neglect, and environmental factors changed him from a sad man who let people walk over him to a violent killer. Perhaps the Joker could be given a childhood backstory similar to Thanos that shapes an audience's understanding of his motives like Todd Phillips's Joker movie did.
Like the Joker, Thanos would go on to kill several more people (even those on his home planet including his own mother) for either entertainment or discovery. Though Thanos claimed he would stop murdering, he seemed to have an uncontrollable urge to do so. This is similar to how the Joker seems to act out in criminal ways in an almost uncontrollable tick-like way. The Joker has never been given an established background or origin in the comics, so his past is very much open for interpretation.
If the Joker is saved for a later reveal like Thanos was, or even shown behind the scenes in the shadows leading up to the franchise finale, it could give the DCEU the same crowd drawl that Thanos gave the MCU. The next DC movie installments could give snippets of the Joker's early life to prepare audiences for a larger plot—one final end all Joker moment that could be as epic and high stakes as Thanos's send-off was. Though it may be difficult to compete with a story centered around the sudden elimination of half of the world, the Joker could be given a climactic plot point with as much urgency and importance.
It is safe to say that the Joker is DC's most famous and well-liked villains ever made. It would be foolish to not include him in the new DC movies. His character is too likable and powerful for him to be utilized minimally. He needs to be the center of a major DC universe plot as the antagonist. That's not to say DCEU needs to make an outline of MCU movies and plot in their own characters, but if the Joker isn't the Big Bad of the DC world, it will definitely disappoint fans of the comics and movies.
Thanos has a very intense background which makes his view of death tragic, but understandable. He seems to also suffer from trauma that has gone untreated. Thanos and the Joker are already quite similar, which is why the Joker would make the obvious choice for the main antagonist in the DCEU. The Batman has already been confirmed to be a trilogy. But if it hopes to stand up to Christopher Nolan's trilogy, it will need to separate itself and make its own identity. That is why the Joker should have his origins set up within the trilogy, but have his plan revealed later down the line in the DC movie universe. An epic battle between the Joker and all other characters in the DC Universe would be much sweeter if the movies leading up to his reveal foreshadow the bigger threat.