CKEditor: consider the new Autocomplete plugin (introduced in 4.10) for addition to core.

https://ckeditor.com/cke4/addon/autocomplete

This plugin provides contextual completion feature for custom text matches based on user input. Every time the user types a pre-configured matching character, such as "@" or "#", they get information about available autocomplete options. It includes the following features:

• Customization of the text matching function.
• Asynchronous data feed.
• Templating system for both the dropdown options and accepted output.
• Throttling to reduce text check frequency.

This feature is a base for implementing specialized autocomplete features, such as mentions, emoji or custom implementations.