Basing this development on the old Play! 1, would be really strange. I’ve worked with Play! 1 and Java and Play! 2 and Scala, and the latter was far better. Actually the design of Play! 1 is a bit strange: for example Play 1 uses exception to achieve a bit more “Ruby on Rails feeling”. And, although Play 1 is still maintained, it is definitely not the future. So, I think the development should definitely be based on Play 2.
Someone doubted that Play 2 is mature enough. It is. I’ve developed a Play 2 application over three years ago and even then was the framework solid. The application is still running without problems.
I wonder what build system should be used. Play 2 is heavily based on SBT, so a Kotlin plug-in for SBT or an alternative build system like Gradle would be necessary.