API Cleanup Ideas

Commands, Menus, Key bindings

Use add() method name for all three main APIs. Simplify Menu method names to remove unneeded "menuItem" part.
Expose menu hierarchy directly, e.g. Menus.viewMenu as opposed to Menus.getMenu(Menus.AppMenuBar.VIEW_MENU).
Expose set of commands directly, e.g. Commands.fileOpen as opposed to CommandManager.getCommand(Commands.FILE_OPEN)

Allow adding listeners with a direct .on() as opposed to $(...).on()
Proxy objects ("CurrentDocument," "ActiveEditor," etc.) that you can attach listeners to once, instead of writing boilerplate code to attach & detach listeners as the current/active Document/Editor changes.
Events for idle document-change processing (either a debounced delay after typing pauses, or on open/save/refresh as linting works today). Useful for Find in Files results updating, real-time linting, possibly some live development functionality, etc.

Generic UI injection-point API -- more robust than current approach of using CSS selectors & jQuery, which easily breaks if we refactor our DOM hierarchy.

Inline editor class hierarchy may be confusing
Inline widget constructor vs. load() method confusion (they are always called synchronously back to back right now? -- but the provider is expected to call both itself, so this must be reimplemented by everyone)

QuickOpen plugin format uses confusing method names: search() vs. match(); itemFocus() is not actually related to focus, etc.