Of course, one could already master with Sonar easily in terms of doing the actual audio mastering and comping everything together with fades. I actually use Sonar to do all of my crossfades and make my final DDP waves... laying out each song on a separate track so I can easily see the overlap and transitions and then bouncing the wave. You can then separate the final bounced wave where you want each track to begin and then bounce each track again for creating gapless playback CDs or MP3s. It's actually really great for various tricky situations... if say you've already mastered everything, but you want a crossfade between two songs and that creates a new peak above 0. After you've bounced the crossfade (with 32bit floating point bit depth of course) Just separate the offending section (crossfading the beginning and ending with the rest of the song) and stick a limiter on just that section. Bounce again... Fixed! I love how flexible Sonar is for this kind of stuff.
One could easily use all of the included software already for the audio side of things, although I don't think you'd get truly top level results. Companies like DMG, iZotope, Fab-Filter, Flux, Sonoris, Acustica Acqua, etc. etc. all make very nice tools that can get you in the realm of professional quality (if you know what you're doing and you have a proper listening environment). I rather not have Cakewalk trying to develop tools that most likely aren't going to really compete on that level anyways... there are already EQs, Comps, limiter, etc. included that are fine. All basic audio tools are already there!
But what could Cakewalk include that would be really cool and/or allow someone to stay in Sonar and deliver a master to a manufacturing plant?
An ISP (intersample peak) monitor. Currently cakewalk would not detect if they're was (or technically - going to be) an ISP, which is important tool these days in mastering. I'm currently using DMG Dualism but it would be nice to have it built right in to Sonar.
A LUFS meter. Essential for people doing broadcast/post work and hopefully the eventual standard for getting levels in a post "volume war" era of audio mastering (if that ever happens). There is already a lot of discussion about Spotify and iTunes radio creating built in volume controls based on a particular readings in LUFS... perhaps we'll really see an end to the volume wars some day when having a loud volume isn't actually louder than everything else?
DDP creation. If you really want to do things correctly, almost all manufacturing plants are using DDP files for CD duplication. A dedicated DDP creator would be great. You could even make it an automated process. Whether or not you have each song in different lanes of one track or different songs spread across multiple tracks feeding into a single bus or output - just select everything and click make DDP (or have it in the bounce to track dialog)... Sonar could automatically render the final wave and reference track start times, include all crossfades or add silence if there was a gap between two tracks. Ability to edit all of this, swap out tracks, manage all of the text/metadata or add ISRC codes would be needed as well.
Sample Rate Conversion! Yup, cakewalk should license iZotopes SRC (hopefully allowing the variety of custom settings) and allow SRC with a session/project... ie convert a project to a new sample rate. Most Daws can't do this within a session, so it would be pretty amazing. Heck, I would LOVE to be able to pull up a 44.1kHz session and convert it to 88.2 or 96. That would be incredible!!!
A proper wave editor. When I double click on an audio clip I don't want to go to a loop construction view... I want to go to Sonar's new audio editor. That has tools for fixing digital errors, clicks, tape hiss, vinyl pops and scratches and maybe even a de-clipper. Powered by iZotope? Even better! Offer a free light version of RX and turn the audio editor view idea into the new iZotope audio editing Region FX. I'm really excited just thinking about the idea... an iZotope RX Region FX... please!
There is always more, but off the top of my head these are the things that would make mastering completely in Sonar a go...
Take these ideas to the Bakers Craig... make a good presentation... we're counting on you! ;-)