Well...I don't know how people in the Cakewalk mothership would answer, but
personally, here is what I would like to see (don't get mad at me, Andrew).
Game-changers are cool, but also disruptive - which can be good or bad. There are still aspects of X3 I haven't fully explored. Getting all that stuff at once was great, but a bit overwhelming. I'd rather be overwhelmed than underwhelmed, but I'd also like to feel I've made maximum use of Addictive Drums
A lot of people have expressed the thought that X3 is fine and since they're still learning it, they're in no hurry for X4. Reading between the lines, I interpret that "I don't really want to be disrupted right now."
The X-series framework is quite mature. Sure, there could be many tweaks, but I don't think you're going to see the single-window interface going away, or the browser turning into something completely different. If it ain't broke...
In my ideal world, I'd look at X4 and say "Doesn't look that different from X3." But then as I worked with it, I'd find lots of surprises...cool tweaks, new features that didn't get in the way of using old features, faster/alternate ways of doing things, etc. For an example of what I mean, I think Apple did a fantastic job (bugs notwithstanding, of course) with iOS 8. It remained 100% familiar, but if you peeled off a layer, there were a zillion new things, all of them easily accessible. I think you're going to see more of this kind of thinking as platforms become more mature.
There's always room for improvement, but it doesn't have to hit you over the head. Actually the Browser is a good example. It was really quite basic at first, but Cakewalk quietly kept improving it. They never made a big deal about it, presumably because it wasn't a "sexy" feature, but those changes have made the difference between "Oh, I guess there's a browser" to "Wow, this is really useful!" That's why I did the browser tip in the Tip of the Week thread: I had a feeling a lot of the improvements went right past people.
Now imagine if that happened across an entire program...that's what I'd like to see. Essentially, better and more innovative ways to do what we're doing now that make Sonar even better for music creation. Yeah, and new features too

And a hovercraft...