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. 

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.

Yeah, make Midi Editor at first.

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!  

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

Trim the final product to size. I'm sure you can do this in Sonar, I just don't know how to do it easily. I get everything all set up in Sonar and then bounce the song to a WAV file. That gets opened in Sound Forge to trim it down for gapless looping. If there's a quick and easy way to do that in Sonar, I don't know what it is.