It's interesting wondering what if's. Thinking about it would the Initiative storyline have been used if Angel and Cordy had come back. If Angel came back, Riley's character would have been less developed, he would have become more like Forrest's character, just another person working in the Initiative. So, would that storyline have been as good if Buffy hadn't had any ties to the Initiative. Without dating Riley, Buffy would never have been invited to join the Initiative, so she wouldn't have learnt about Adam. The storyline would have been pretty pointless.