Mashup builder is not yet available in NextGen Composer but I believe it would be an opportunity to implement a couple of improvements that could help a lot the understanding of the mashups when we are changing or maintaining applications that are already created.
What do you guys think?
Great ideas. I've passed along the first item about comments/descriptions for why a service is included in a mashup to the right team.
For the second item, the "Where used" capability is sorely needed across the platform, including visualization. I am targeting a feature in an upcoming release to address this also.