A modern programming language that makes developers happier.

kotlinlang

Ideally I don't want to have two IDEs open, IntelliJ CE has enough Android support already iirc

I have faced this pain also. I use two IDEs, but not at the same time. When working predominantly on pure kotlin modules (no Android APIs) I use Intellij, and when working on the Android app I use android studio, but also use softlinks to the pure kotlin modules, and a separate set of gradle files. I seem to be getting away with it so far. I can make quick changes to the kotlin modules from Android studio when I need to, and it all compiles fine.