Of course, the Crisis on Infinite Earths storyline featured the death of Supergirl — which, it seems safe to assume, will not occur on Supergirl.
But, in one new scene, Superman and Supergirl are seen replicating the pose of the characters from the 1980s DC Comics crossover Crisis on Infinite Earths.
If the Arrowverse were to adapt a version of the Crisis on Infinite Earths storyline — as they recently have with Flashpoint — it would be a means to merge Supergirl with the Arrowverse.
However, the new extended trailer for Supergirl season 2 teases a comic book crossover event that did result in worlds merging.