Kotlin is designed to be very "compatible" with Java, so some plugins (may be many of them) can work both for Kotlin and Java with almost no additional code. But to enable this support there must be some simple layer to access to Java representation for Kotlin structures.
For example: in my case this plugin is ‘wicketforge’ that automatically generates HTML templates for Java classes using superclass as a semantic marker. What more important it enables fast navigation between these HTML files and Java classes and this operation is used very often during development (similar to switch between C++ header and source files). To make this plugin to work with Kotlin too all I need is to get superclass for my Kotlin object.
But when I install ‘wicketforge’ plugin it depends on ‘properties’ plugin only. I believe a lot of plugin owners will be reluctant to add new dependencies for small patches.
So if there is a chance to move some minimal Kotlin PSI support to the general purpose ‘properties’ plugin it will make very simple to persuade plugin developers to accept patches for Kotlin.