It would be great to route tracks from cubase into H4 for filtering and processing then back out
This is completely possible, but would involve Steinberg to finally implement sidechaining into instruments, something they have put no effort into doing unlike just about every other DAW out there. I have been waiting for this feature for years now for my Virus Ti which also supports sidechaining into instruments for processing. Logic, Reaper, Sonar, Live all support this.
My bet: they will introduce the fully revamped “ProTools-level” audio engine (with flexible mixer routing, sidechain, etc.) in C7 only 