Kotlin 1.2 has been released and provides useful syntax enhancements, faster compilation and multiplatform support. That's the version we are likely going to use for Spring Boot 2 and start.spring.io as well, so let's upgrade to it, potentially using -api-version 1.1 and -language-version 1.1 to keep good compatibility with those who stick to 1.1.
I have asked Kotlin compiler team confirmation that it is safe for Spring Framework to build binaries with Kotlin 1.2 with apiVersion = 1.1 and languageVersion = 1.1 option enabled while keeping compatibility with Kotlin 1.1 projects. The compatibility documentation mentions "OV compiler may not accept NV binaries (e.g. those that exhibit newer language features or APIs)".
If their answer is no, I think staying on Kotlin 1.1 in Spring Framework 5.0 and leveraging Kotlin 1.2 as of Spring Boot 2.0.RC1 and start.spring.io will be our best option, upgrading to Kotlin 1.2 in Spring Framework 5.1 timeframe.
In both case, I do think Spring Boot 2 and start.spring.io should leverages Kotlin 1.2.