Mmmm... I dont think GC or WPF will be updated in a 4.x release, but thats welcomed anyway.
There are many interesting research or commecial GC implementations in the Java world, realtime, concurrent or super large scale ones.
And Azul has started an Managed Runtime Initiative to improve GC and runtime performance in general, their approach is to adapt managed runtime requirements all the to the operating system and hardware (or virtual hardware). I dont think MS will paticipate this but at least Mono and some external researchers could do some experiments with it.