Dependencies management : global exclusions, more features to analyze their usage, more features to control what we expose and what we consume (skinny wars/ears hell ....) and before all ... many fixes (there are various bugs in aether integration with some plugins like the dependency plugin or some enforcer rules)
Repositories management (more flexible, with a distributed architecture not necessary centralized).
A better design (putting repos in poms is useless, identification doesn't exist) to have something more efficient to retrieve binaries without having a repo manager.
The ability to deploy on repo at the end the build to have a better consistency locally and remotely.
The ability to launch several maven builds in // without breaking the local repo or without having to have one local repo per project ....
The ability to manage various POM versions (and if possible formats but myself I'm not fan) within repositories
Settings management (users, organization, entreprise, project - various level and with the ability to share them)
Profiles management (At the same time to have something more strong to validate that a default build is running and more power in profiles activation/deactivation)
POM : Mixins, less verbose XML (or others formats), new elements (encoding, global excludes ...)
packaging -even if not core related - (when you need to go further than a jar, war the assembly isn't enought. It should support a scripting langage like groovy to easily create custom packages ... but the heart of the problem is also often the transitive dependencies management)