This could mean some interesting things for the future slate of DC Comics films of the future; none of which are spelled out in any post-credits scenes (Though Harley Quinn does try to tell the audience something interesting about Batman before she’s cut off by the studio logos).
That could also mean that Harley and Cassandra Cain’s new joint venture isn’t meant to last terribly long either.
Between her loose cannon personality, and the fact that she’ll be working with the Suicide Squad again, Harley’s ways will probably see Cassandra in need of somewhere else to hang out fairly soon after the events of Birds of Prey.