Is there some kind of "backlog" of Scala's design issues or mistakes, which are planned/not planned to be solved in a future release?
I know that there is Trac and the SID process, but in both cases you only see the changes after they have been made.
Maybe I miss some existing infrastructure where language/specification changes can be collected and discussed?
We "paid" most accumulated technical debt in 2.8. We maintain no list of design mistakes that are scheduled for changing now or in the future. In fact I think we'll see a pretty stable phase for the next years where most energy will be spent on getting great APIs and libraries and figuring out the parallelism challenge.
That kind of information would be more readily available at the Scala forum "internal" or Scala Debate, although those forums are in the process to being moved to google-groups forums:
Soon: Google groups scala-internals.
Right now, the only information on Scala 2.9 is this roadmap (august 2010):
One major addition for this version will be parallel collections.
Parallel collections will reap the benefits of the new uniform collections framework in 2.8. They will integrate seamlessly into that framework, supporting the same operations as Scala's sequential collections, but making use of multicores to speed things up.
The first version of parallel collections is already in the nightly builds, so you can experiment with them today.
Beyond parallel collections and Scala 2.9, we expect lots of interesting developments for making use of massively parallel hardware in novel ways.
Of course, there's always this: http://www.scala-lang.org/node/8579