Nothing really new since this article dates from 2014, but I just stumbled upon it: https://blog.jetbrains.com/idea/2014/03/postfix-completion/
It presents a feature introduced in IntelliJ, which is about snippets / autocompletion.
The goal of this “postfix completion” system is to be able to transform the text that has just been typed.
I let you discover the GIFs embedded in the article which are self explanatory.
This trick would be nice in Kakoune, I think we have everything needed in place (InsertChar hooks, suggestions…)