I wanted to give this a few viewings to make sure I was over that puppy love stage and got a good understanding of what drives each character and the main plot of the movie. If Zemo is truly the villain then his motivations are the main plot of the film. He was affected by the actions of the Avengers in Sokovia. His plan is to tear them apart by pitting their leaders, Tony and Steve, against each other.
The conflict for the rest of the team comes in the form of the Accords, which were formed because of the casualties of their battles around the world -- three of the four examples that Ross names were Avengers missions.
So we have a main plot and a key sub plot both working to create conflict within the Avengers.
An argument can even be made that it's more Iron Man 4 than it is Captain America 3, since the plot moves along based on his decisions brought on by the guilt over Sokovia/Ultron, which was mainly his fault, and his reaction at the very end. He starts the battle at the airport and in Siberia, and he's the reason Spider-Man is even in this film in the first place.
I'll go a step further than the original question and say that this movie is Avengers 3, not just 2.5. They are the main characters and the plot is driven by their actions and the actions of their villains -- Ultron in Sokovia, Loki in New York, Ross with the Accords, Zemo with Vienna and Siberia.