There is a reason that Ursula and Cruella were brought on and it's genius if you just ignore the fact that they advertised them as big bads. If you focus on what they were, not what they could be, it's an excellent plot.
Rumple brought the Queens of Darkness together because it could turn Emma dark. Not because he needed a trio of evil to do what he could easily do himself (besides getting into Storybrooke). He needed Maleficent to get Emma to turn against Snow and Charming, because she would be unable to forgive them and start getting angry, which, as we know from Star Wars, leads to the dark side. He needed Ursula to exact her revenge on Hook, because he's Emma's true love and losing him would make her hate Ursula, and hatred too is the way to the dark side (unfortunately for Rumple, Ursula didn't kill Hook. Thanks Ariel.) He needed Cruella because he knew she was an awful person who would do anything to kill the author, but she was defenseless and if Emma killed her, it would turn her heart dark just like it did with Snow White when she banished Lily.
The characters were brought in for excellent and creative reasons. It all ties into the same plot, too. It's all turning Emma dark. We know the savior can somehow write happy endings in the LWM (likely to be explained in the finale). He needed to turn her dark so that she would write evil happy endings. Queens of Darkness? Manipulation on Emma (as I explained above). The Author? Manipulates Cruella into doing her awful stuff which leads Emma to kill her (so he's just the Queens of Darkness). The only exception is Zelena. She's basically completely disconnected from the plot. This leads me to believe she will cause something that effects Storybrooke in Season 5.
I think that this is one of the best played arcs yet. They really aren't trying to focus on multiple characters (which is somewhat of a problem, because that means no Ruby-type characters, but also means we won't have small disconnected backstories like we did in 4a), it's all development on our mains (Ursula brought in some Hook, Zelena and Marian gave Robin his first real backstory episode, Maleficent brought in Regina and Snowing stuff (and I believe the Snowing stuff is a great sign of the gray line and is played excellently), Cruella brought character development in to Emma by seeing how she fights with the growing darkness, as did Lily, etc.) I love this arc, and I hope Season 5 is played just as well.