WCM, Sling and JCR Helpers
Most of what is listed here is either implemented as part of http://wcm.io/ as small helper methods or sling/wcm commons functionality - or obsolete/superseded by Sling and AEM functionality.
Application Helper Features
Prio 2 Page object cache infrastructure extensible by page object readers
Perhaps no longer relevant for configuration infrastructure (see Multi Tenancy and Configuration), but it may be still required for other usecases
Later DOM HTML API and Java-based Component Model
If we still require this type of components
Prio 2 Content types/Mime types, File extension
Prio 2 Escape helper methods
Prio 2 URL Parameter parsing methods
Prio 2 Version information: Make the version(s) of the deployed application accessible from the outside via (hidden) URL
WCM Helper Features
Prio 2 Common page and resource filter implementations
Prio 2 Helper for ignoring well-known AEM properties with special meanings, or resources with it (e.g. from MSM that hides components that are deleted with an MSM inheritance, but still present in the repository)
Prio 2 htx -> html redirection
WCM Helper Templates
Prio 1 Text Resource Template
Later "Directory Upload" Template - Supports uploading a ZIP that is extracted into a folder structure - supports uploading of whole microsites with static HTML and resources or other mini frontend-only applications that can be references via their URL
Prio 1 "Structure Element" Template
Later "Smart Activate Tree" Template - Does not single page activation, but packages a content package with all relevant packages, activates the whole content package to the publish instances, and marks the relevant pages as activated
Sling Helper Features
Prio 2 Helpers for detection resource modifications and set not-modified-since headers
Prio 1 Abstract default implementation of background service with best practices
Avoid long-running JCR sessions
Do not use administrative session
Cluster/topology-aware
Provide dependency injection
Prio 2 Helper classes for resource types, run modes, Sling URLs
Check which of these features are available in Sling API as well, or where Sling API may be fixed or extended
Proper support of run modes, remove old-style runmode support already deprecated in Sling/CQ
Prio 1 adaptTo extension for "adaptNotNull"
Prio 2 Sling Suffix handler infrastructure (maybe with an API that is easier to understand)
JCR Helper Features
Later Helper methods for handling binary nodes and mime types inside JCR