I didn't think Whedon dwelled too long on any one of those dream sequences, and they each served a purpose. Thor's dream was for the Infinity Stones reveal and to tease Ragnorak, nothing more than a lazy Phase 3 set-up if you ask me. Cap's dream actually played into moments of character development throughout the film. It helped him realize he has no purpose outside of fighting a war, you see it when he becomes uncomfortable going back into Barton's house, and he even admits it towards the end of the movie to BW. Widow's dream goes to show she's been turned into a monster, and sets up her inner conflict for weighing her responsibilities as an Avenger or giving it all up to try and live a normal life.
Given Marvel's penchant for long-form storytelling, I presume Steve and Tony's developments in AoU would play into their conflict in Civil War. Registration act aside, there's also some headbutting in selfish needs here - Steve is more interested in the process of ending conflict, whereas Tony is seeking the actual end of conflict. As much flack AoU got from the fans, Whedon and Marvel did squeeze in some good character development pieces throughout the film, though they didn't place a whole lot of focus on them.