I’m not sure Darla and Angel would have ever worked. Liam and Darla as humans would probably have worked, just as Angelus and vampire Darla worked perfectly. But I don’t see souled Angel and vampire Darla working, she takes so much pleasure on being evil and little willingness to impose restrains on herself (I’m actually kinda of surprised that she didn’t attempt to kill Lindsay after his jealous outburst). Human Darla is intriguing, but we really don’t know much about her. I assume that being a prostitute jaded her beyond repair, she rejected salvation right before the Master sired her. Newly human Darla was still scheming and manipulating before the soul kicked in. And she felt remorse, but her fear of dying was too strong. I’m not entirely convinced that she wouldn’t have tried to become a vampire again once Angel had left her alone. I doubt that Angel would have ever overlooked that type of behavior.
But they did look great together. Angel has type: Buffy, Darla, Nina, Kate, Eve... Cordelia was the only one that wasn't short and blond.