Harley Quinn doesn't fill any of Babs old roles in Birds of Prey at all in any form. Unless you mean reasonably well known character with more casual audiences not as familiar with the other members. Which doesn't seem to have worked out given the generally poor reception of the Harley taken over Birds push. Harley taking them over hasn't been beneficial at all.
Which is kind of the problem. I'd much rather have Birds of Prey focus on Black Canary and Huntress as protagonists(and I'd like Zinda Blake to be brought back.) than be treated as Harley's sidekicks.