All good observations. That is exactly why I wrote about the libraries used by it and not about setting itself as a platform library.
Coincidence or not, the two libraries/frameworks you mentioned, Akka and Play are both essential dependencies to Lagom and on a side note, Play 1.0 was also written in Java.
But again, my point here was not promoting Lagom or anything, but just to start a discussion about strategy using some publicly available project on the JVM ecosystem with a wide enough scope to use as the tip of the iceberg of features that one could expect to see covered in something to be called a platform.
The topic could be renamed to "Review the dependencies of a relevant project and check if or how much should be covered by the platform" and maybe that would still be a simplification of what I'm talking about.
I completely agree that we should not forget about Scala.js and maybe even Scala Native should count some points, but I would keep the options open about picking the best in class for each runtime, as some browser specific library might be the killer feature for Scala.js and at the same time irrelevant for a Scala server side.
You are right about not being possible to call Java from Scala.js, and even beyond that, due to some inherent differences in the underlying platforms AFAIK related especially to reflection and thread models, even pure Scala projects might have difficulties compiling to other runtimes, thus we might need to select an alternative library or a port (I haven't used yet but I know there is an Akka.js project for instance).
About Akka and Play being part of the standard library, I believe scala actors where removed from the standard library as Akka became the de facto standard. Both are not standard library, but integral part of Lightbend Reactive Platform, which could actually be another target to the Idea I was trying to convey.
Another possible list to be checked is Awesome Scala, besides a couple of others I've seen in this discourse instance. At the end the main point is identifying what are the expected elements to be part of the platform and create separate topics to discuss them.
By the way, so people can easily find where they can contribute to discussions or suggest apparent missing things, is there some centralised list available somewhere showing the status for different modules of the scala platform (proposed, in review, etc)?