The following two unimplemented SE proposals are fairly impactful on Swift’s core programming model:
SE-0155 Normalize Enum Case Representation
SE-0169 Improve Interaction Between private Declarations and Extensions
Both proposals were only recently accepted, but would be good to get in for Swift 4. Unfortunately, the engineers on my team at Apple are fairly saturated in the near term with other tasks. If there is any interest from members of the community in taking on implementing (part of) either of these SE proposals in the near term that would be extremely welcome.