Warning! The following contains spoilers for the second part of the "Elseworlds" crossover. Read at your own risk!
The Arrow-verse crossover "Elseworlds" really ramped things up in Part 2, and in addition to lots of Batwoman goodness and Gotham City, there was a massive reveal that potentially dictated the future of the Arrow-verse. The heroes ended up meeting The Monitor, who congratulated them on passing the first test of stealing the book from Dr. John Deegan. It turns out the whole thing was a test, and will prepare the heroes for what sounds like Crisis On Infinite Earths.
As some had predicted, The Monitor appeared in "Elseworlds" in order to test the might of various Earths' heroes to see if they were game to take on a far greater threat. Barry, Oliver, and Kara did better than Earth-90's heroes, but The Monitor wasn't adequately satisfied with the level of challenge John Deegan created for them. The supreme being teleported back to the deranged Arkham Asylum doctor and demanded he craft a more challenging scenario for the heroes.
The result was Barry and Oliver being completely stripped of any superspeed, and transformed into Gotham City villains. The two manage to escape arrest from GCPD, but their escape is hampered by Superman in all black. The Man of Steel still errs on the side of justice apparently, but his seemingly tyrannical nature doesn't quite make him the Boy Scout Kara knows as her cousin.
With Barry and Oliver without powers, and Kara shown in a trailer to be locked away by her seemingly evil cousin, this scenario should be much harder than the first scenario. The good news is that if the heroes succeed, they'll be deemed worthy of having what it takes to battle the massive threat even more powerful than The Monitor. The bad news is if they fail, The Monitor is probably going to destroy their world.
Of course, fans should have a sneaking suspicion the Arrow-verse heroes pull through, considering each show will be returning in 2019. Additionally, it's known that Barry lives long enough to disappear in some massive battle that involves all the Arrow-verse heroes in a couple years in an event that some have speculated is the Arrow-verse version of "Crisis On Infinite Earths." Now, The Monitor has provided strong evidence that is the case, and that something crazy is on the horizon.
Specifically, that something crazy is that the Arrow-verse is intending to feature "Crisis On Infinite Earths" sometime in the future. Whether that will be next year's crossover, the year following, or even three years from now is unknown. That said, the fact that the writers have openly acknowledged via an episode that The Monitor is preparing the heroes for an oncoming threat is huge, and possibly a sign this event could come sooner than later.