I thought I'd throw this out there for all of the Sonar people that might be scanning the forum for ideas to improve their product and the Sonar experience. 
What would put X3 over the top is a Mastering system built into the program much like what Presonus has done with Studio One Professional. 
I had been using the T-Racks program.  I like the fact that all of the tracks stack up and you can compare easily.  There are a lot of good features.  However, the downfall is that you can not use 3rd party plug-ins nor can you use external gear.  So I have been making do with X3 as a mastering tool.
Yes, I know that Mastering should be left to people that do it for a living but I'm just having fun. 
I've toyed with the idea of buying Studio One Pro at some point.  I've heard and read very positive things. 
 
So what do you think, Cakewalk?   Anyone?  Anyone?  Bueller?

A Mastering "area" would definitely be beneficial, even if for no other reason than allow a mixing Engineer to see if there may be any potential problems in Mastering later. I currently use Wavelab for my Mastering purposes, which is a whole animal when it comes to Mastering, but at times, I want to produce a quick estimated demo for bands without having to export the mix to Wavelab and Master a demo for playing in a car.
 
The only 'real' issue I have with Mastering in Sonar is that Sonar doesn't have ample metering for this. You would need Algorithmix's DR meter, or the one from Brainworx to even approximate loudness, or, go in for Waves WLM or Flux's metering solution(s). An integrated burner with Meta-Data would be awesome, too.
 
I've suggested that Sonar could make a more Pro-Line Sonar, that would rival Pro-Tools HD and/or Nuendo. It could be called Sonar-Professional, or Sonar-Industry, which the name would exude the perception of "this is the package the studios buy".
 
There's an enigmatic perception in the audio industry that the more expensive something is, the better is is. A $10,000 power cable is amazing, and you can TOTALLY hear the difference (no, not really (read my signature))! BUT, that marketing happens to work. However, it doesn't have to be that ridiculous.
 
Sonar X3-Industry, for example, at $1195 street price, includes all the features Cakewalk and users would love to add, including a DDP+ Mastering suite, and change the game....no...change the NAME of the game!

Studio One Pro's mastering page is not quite the same thing as Wavelab or Sound Forge, its main strength is being able to do album assembly very well. In that sense, it's more like Sony's CD Architect on steroids than something like Wavelab.
 
For mastering individual cuts, you can do a lot in Sonar. The LP processors are really good in that respect. There's no multiband "maximizer"-type processor; I believe the Concrete Limiter is stereo. However, you can come very close with the LP Multiband.
 
I stick Ozone in there for metering and traditional maximizer functions, along with the (free) SSL X-ISM plug-in meters for checking on inter-sample distortion.
 
That said, Sonar is not that far away from being a mastering program...doesn't seem like it's out of the question for a future version.

Studio One is one of the few major DAWs I've actually never seen in action, so excuse me if what I'm saying is exactly what it already does, but...
I would like to be able to press a button after I feel happy with my main mix, that automatically bounces my project, just gives me a large single track view of it and loads up a bunch of good meters and perhaps a user-definable set of mastering plugs (defaulting to the Sonar offerings). Too often I find myself doing this manually. It would also help to be able to "group" projects/songs for this purpose, similar to what Ableton does, to open a couple of bounces in a single project for cd building or comparative leveling.

These are all things that are already possible but are done very frequently by hand. It would require some rethinking about project organisation and would need some major new feature programming on Cake's part, but those are mainly concerning automating existing processes. Would love this.

It doesn't take much to export a mix and create a new project with Ozone* slapped on the master channel. Infact you could create template for mastering and load that for finishing the exported mix.

Grum.

* Or whatever your choice of software is.

Sanderxpander
Studio One is one of the few major DAWs I've actually never seen in action, so excuse me if what I'm saying is exactly what it already does, but...
I would like to be able to press a button after I feel happy with my main mix, that automatically bounces my project, just gives me a large single track view of it and loads up a bunch of good meters and perhaps a user-definable set of mastering plugs (defaulting to the Sonar offerings). Too often I find myself doing this manually. It would also help to be able to "group" projects/songs for this purpose, similar to what Ableton does, to open a couple of bounces in a single project for cd building or comparative leveling.

These are all things that are already possible but are done very frequently by hand. It would require some rethinking about project organisation and would need some major new feature programming on Cake's part, but those are mainly concerning automating existing processes. Would love this.

A similar workflow:
 
1. Bounce your mix to a track in your project, and save your project. The mix will now be saved in your project's audio folder (assuming you save a project into its own folder).
2. Close the project, and open a new template project that has all the desired mastering plug-ins and screen layout you want.
3. Drag the mixed file from the project audio file (access it via the browser) into your mastering template.

In the survey that Cakewalk put out before X3 came out, one of the questions asked would we like to see the ability to master possibly in a future version. I gave a resounding yes, as I have coveted how the Studio One  Mastering section works and would love to see this in a future version of Sonar. I would also like full fledged wave editing inside of Sonar for a future release.

Anderton
....... 
That said, Sonar is not that far away from being a mastering program...doesn't seem like it's out of the question for a future version.

Looking forward to that!

Since the OP mentioned Studio One as an example, I must point out that there's one area Sonar is definitely inferior to it, and it's a core feature all users (not just Producer users) will benefit from. It's.....timestretching. After you recorded all your audio parts, you realize you want to change the tempo of the entire song and want all audio clips to timestretch (follow the change) at once. In Studio One, all you need to do is enter a new tempo in the tempo field on the control bar.

So, Cakewalk, if you're reading this, postpone the idea of adding a mastering system until X5 or later.

Jlien X
Since the OP mentioned Studio One as an example, I must point out that there's one area Sonar is definitely inferior to it, and it's a core feature all users (not just Producer users) will benefit from. It's.....timestretching. After you recorded all your audio parts, you realize you want to change the tempo of the entire song and want all audio clips to timestretch (follow the change) at once. In Studio One, all you need to do is enter a new tempo in the tempo field on the control bar.

So, Cakewalk, if you're reading this, postpone the idea of adding a mastering system until X5 or later.

I don't recall time stretching being in the survey, so it might have to wait until X5, or a time stretching thread.