I use Goldwave audio editor.  If I were better with Sonar I could probably do most of what I do in Sonar but I'm not and I'm familiar with Goldwave so I use it.

Soundforge for MP3 conversion, topping and tailing, analysis, repair, noise reduction, along with CD Architect for Red Books CD creation plus a host of other useful things that escape my furry mind at the moment. 

In Sound Forge it's easy to find a precise spot for editing. Ctrl+Shift+K plays a couple seconds up to the Now time. You can then use the arrow keys to move it in tiny increments to find the exact place you want. Doing this with a mouse is very clumsy in comparison.
 
I also use it for noise reduction and mp3 conversion.

First I would do a spreadsheet of the current wave editing features that are included in some of the other major DAWs ie Cubase ,Logic,PT and Studio One.What do they offer that Sonar does not..where do they overlap in wave editing and where they differ..I would start with the features they all share first then work from there. There area few mentioned in this thread that all seem to overlap and those are a good starting points also.Pencil editing for one.All for the changes as long as Sonar doesn't try to be too many things at once I am afraid other things would suffer or be pushed back in the pipeline.Just my two cents.

synkrotron

Benny Bear
I love Sonar and everything it does, but just at the end of the process I like to get out of Sonar (into Sound Forge) to top and tail, mp3 and get a slightly different perspective. No criticism just a habit! I was using Sound Forge a long way back and so am used to it.

I could have written those words myself, verbatim...

Ditto with Audacity here.  If I want wav and mp3, it's a lot easier to export a wav out of Sonar, then drag/drop it onto Audacity.  Top/tail, fades and a quick export as wav followed by quick export as mp3.
 
I don't think Audacity does any of this better, but the steps go faster for me.  The alternative in Sonar is to fiddle until the selections are correct, then bounce the entire mix to a track.  Then top/tail, fade, solo, go thru the export process for wav, then for mp3 ... and BTW, I have yet to figger out to get MP3 options set as presets/default.
 
It's just slower and more fiddly to do in Sonar.
 
PLEASE! do NOT spend precious dev time on an integrated editor.  At least not until the many other things that need attention have had their turn first.
 

Yeah, make Midi Editor at first.

Dan Gonzalez [Cakewalk]
What do they do that SONAR just can't do?

In addition to what's already been mentioned:

• Assemble/edit multi-channel Wav files
• Advanced CD layout/burning (crossfades/etc)

 

Back out of dis-interest with sonar for a good question:  
 
SoundForge, and to a lesser extent Samplitude:  SPEED.    Sonar is an interface for slowpokes, though it does many good things.   It has so many foolish idiosyncracies:   just one out of countless for instances, wouldn't it be normal to set a loop begin point with the keyboard and then hit tab, or enter, or Control-ZAlt7 or something to get to the loop end point?   No.  You have to reach for the mouse.     SoundForge just works faster, way way faster.    I edited nearly a thousand wave files in a couple of days recently, and it would have taken weeks in Sonar, even though what I was doing was not particularly complicated (pitch shifting, loop making, adjusting crossfades).   Sonar, use the PRV window to look at controllers, you have to spend a minute of precious time closing unwanted controller lanes and you have to do it over and over ...
 
So before adding new wave editing functions, make sure that sonar can be used FAST which means with a computer keyboard and without visual clutter, especially without clutter that has to be opened or closed repeatedly.
 
And good grief: how many menus does SoundForge have?  One.   How many does Sonar have?  Who can count them?  Or even find them all!  

Samplitude Pro X for mastering, it works really well, CD Architect on steroids!

Am happy you're thinking about this Dan. 
 
Personally I like having Wavelab for metering and precision waveform editing. I'm not sure Cakewalk should even try and do all the stuff wavelab does- there are so many possibilities in DAW land.
 
One request: an easy way to add audio editors. Project 5 version 2 had a dedicated button for such and I wish sonar had the same